feat(skills): rename "agent-browser" skill to "core"; make CLI-served main skill actually useful (#1253)

Before this change, the main skill served by the CLI (`agent-browser
skills get agent-browser`) was a ~40-line discovery stub whose content
was essentially "run `agent-browser skills get <name>` before doing
anything." Agents already inside the CLI got no signal from it — the
content they needed to actually use the tool lived only in the `--full`
references.

Split the two jobs apart:

- **`skill-data/core/`** (new) — the runtime usage guide. 420-line
  `SKILL.md` covering the snapshot-and-ref loop, common workflows
  (login, extract, screenshot, multi-tab, sessions, iframes, dialogs),
  waiting strategies, element selection strategies, troubleshooting,
  and when to load a specialized skill. Supplementary `references/` and
  `templates/` (moved from `skills/agent-browser/`) provide the full
  command reference under `--full`.
- **`skills/agent-browser/SKILL.md`** — still the discovery stub that
  `npx skills add` installs, now marked `hidden: true` so it stays out
  of `skills list` inside the CLI. Body is a clean pointer to
  `agent-browser skills get core` and the specialized skills.

The `hidden: true` frontmatter flag is a new, general mechanism: skills
marked hidden are omitted from `skills list` and `skills get --all` but
can still be fetched by explicit name. This keeps the stub reachable
for anyone who installed via `npx skills add` without polluting the
CLI-side skill listing.

## Behavior

```
$ agent-browser skills list
  agentcore       Run agent-browser on AWS Bedrock AgentCore cloud browsers...
  core            Core agent-browser usage guide. Read this before running...
  dogfood         Systematically explore and test a web application...
  electron        Automate Electron desktop apps (VS Code, Slack, Discord...)
  slack           Interact with Slack workspaces using browser automation...
  vercel-sandbox  Run agent-browser + Chrome inside Vercel Sandbox microVMs...

$ agent-browser skills get core          # the actual usage guide
# ~420 lines of workflows, patterns, troubleshooting

$ agent-browser skills get agent-browser # still works if called explicitly
# the thin stub, now pointing at `core`
```

External `npx skills add vercel-labs/agent-browser` behavior is
unchanged: it finds and installs the thin `agent-browser` stub, which
tells the agent to run `agent-browser skills get core` for real
content. Version drift protection is preserved — the stub is the only
thing that gets copied; the real content is always runtime-fetched.

## Updated

- `cli/src/skills.rs` — `SkillInfo.hidden: bool`, parsed from
  frontmatter; `run_list` and `run_get --all` filter it. 3 new unit
  tests for the frontmatter parser.
- `cli/src/output.rs` — top-level `--help` and `skills` subcommand help
  reference `skills get core` / `skills get core --full`.
- `AGENTS.md` — "update these files for user-facing features" now
  points at `skill-data/core/` instead of the stub, with a note that
  the stub is not the right place for feature content.
- `README.md`, `docs/src/app/skills/page.mdx` — describe the new
  split and `skills get core --full` as the recommended entry point.
- `evals/cases/{command-usage,skill-selection}.ts` — expect
  `skills get core` in agent output instead of `skills get
  agent-browser`. Eval lib still reads `skills/agent-browser/SKILL.md`
  (simulating what an agent sees after `npx skills add`).

All 11 skills unit tests pass. `cargo clippy -- -D warnings` and
`cargo fmt --check` clean. Verified end-to-end: `skills list` shows
`core` + specialized (no stub), `skills get core` returns the new
content, `skills get agent-browser` still returns the stub on explicit
request.

cli/src/output.rs

@@ -2829,10 +2829,11 @@ rather than relying on cached copies.
 Examples:
   agent-browser skills
   agent-browser skills list
-  agent-browser skills get agent-browser
+  agent-browser skills get core
+  agent-browser skills get core --full
   agent-browser skills get electron --full
   agent-browser skills get --all
-  agent-browser skills path agent-browser
+  agent-browser skills path core
   agent-browser skills list --json
 
 Environment:
@@ -2854,7 +2855,7 @@ agent-browser - fast browser automation CLI for AI agents
 Usage: agent-browser <command> [args] [options]
 
 Start here (for AI agents):
-  agent-browser skills get agent-browser --full
+  agent-browser skills get core --full
 
   Skills ship with the CLI (always version-matched) and include workflow
   patterns, ref/selector usage, and copy-paste examples. Prefer this over
@@ -2862,9 +2863,9 @@ Start here (for AI agents):
   apps, Slack, exploratory testing, and cloud browser providers.
 
   skills [list]                List available skills
-  skills get <name>            Get a skill's core content (overview + examples)
-  skills get <name> --full     Include references and templates
-  skills get --all             Get every skill
+  skills get core              Core usage guide (overview + common patterns)
+  skills get core --full       Include full command reference and templates
+  skills get <name>            Load a specialized skill (electron, slack, ...)
   skills path [name]           Print skill directory path
 
 Core Commands:

cli/src/skills.rs

@@ -10,11 +10,21 @@ struct SkillInfo {
     name: String,
     description: String,
     dir: PathBuf,
+    /// When true, the skill is omitted from `skills list` and `skills get --all`
+    /// but can still be fetched by name via `skills get <name>`. Used for
+    /// bootstrap stubs that exist for external tooling (e.g. `npx skills add`)
+    /// but aren't the intended entry point for agents already inside the CLI.
+    hidden: bool,
 }
 
 /// Skill content is split across two directories:
-/// - `skills/`     — the bootstrap skill (discoverable by npx skills add)
-/// - `skill-data/` — specialized skills (only served by the CLI)
+///
+/// - `skills/` — discovery stubs (picked up by `npx skills add`). Carry
+///   `hidden: true` so they don't show up in `skills list` or `skills get
+///   --all` inside the CLI, since they exist only to redirect external
+///   agents to `skills get core`.
+/// - `skill-data/` — runtime skill content served by the CLI (`core`,
+///   `electron`, `slack`, `dogfood`, etc.).
 ///
 /// Both are shipped in the npm package and searched by `discover_skills`.
 const SKILL_DIRS: &[&str] = &["skills", "skill-data"];
@@ -74,8 +84,8 @@ fn find_skills_dirs() -> Vec<PathBuf> {
         .collect()
 }
 
-/// Parse YAML frontmatter from a SKILL.md file. Returns (name, description).
-fn parse_frontmatter(content: &str) -> Option<(String, String)> {
+/// Parse YAML frontmatter from a SKILL.md file. Returns (name, description, hidden).
+fn parse_frontmatter(content: &str) -> Option<(String, String, bool)> {
     let content = content.trim_start();
     if !content.starts_with("---") {
         return None;
@@ -86,6 +96,7 @@ fn parse_frontmatter(content: &str) -> Option<(String, String)> {
 
     let mut name = None;
     let mut description = None;
+    let mut hidden = false;
 
     let lines: Vec<&str> = frontmatter.lines().collect();
     let mut i = 0;
@@ -104,11 +115,13 @@ fn parse_frontmatter(content: &str) -> Option<(String, String)> {
                 desc.push_str(lines[i].trim());
             }
             description = Some(desc);
+        } else if let Some(val) = line.strip_prefix("hidden:") {
+            hidden = matches!(val.trim(), "true" | "yes");
         }
         i += 1;
     }
 
-    Some((name?, description.unwrap_or_default()))
+    Some((name?, description.unwrap_or_default(), hidden))
 }
 
 /// Discover all skills across the given directories.
@@ -134,11 +147,12 @@ fn discover_skills(dirs: &[PathBuf]) -> Vec<SkillInfo> {
                 Ok(c) => c,
                 Err(_) => continue,
             };
-            if let Some((name, description)) = parse_frontmatter(&content) {
+            if let Some((name, description, hidden)) = parse_frontmatter(&content) {
                 skills.push(SkillInfo {
                     name,
                     description,
                     dir: path,
+                    hidden,
                 });
             }
         }
@@ -198,7 +212,10 @@ fn collect_supplementary_files(skill_dir: &Path) -> Vec<(String, String)> {
 }
 
 fn run_list(skills_dirs: &[PathBuf], json_mode: bool) {
-    let skills = discover_skills(skills_dirs);
+    let skills: Vec<SkillInfo> = discover_skills(skills_dirs)
+        .into_iter()
+        .filter(|s| !s.hidden)
+        .collect();
     if skills.is_empty() {
         if json_mode {
             println!(
@@ -242,7 +259,7 @@ fn run_get(skills_dirs: &[PathBuf], names: &[String], get_all: bool, full: bool,
     let all_skills = discover_skills(skills_dirs);
 
     let targets: Vec<&SkillInfo> = if get_all {
-        all_skills.iter().collect()
+        all_skills.iter().filter(|s| !s.hidden).collect()
     } else {
         let mut targets = Vec::new();
         for name in names {
@@ -490,18 +507,36 @@ mod tests {
     #[test]
     fn test_parse_frontmatter_basic() {
         let content = "---\nname: test-skill\ndescription: A test skill.\n---\n\n# Test\n";
-        let (name, desc) = parse_frontmatter(content).unwrap();
+        let (name, desc, hidden) = parse_frontmatter(content).unwrap();
         assert_eq!(name, "test-skill");
         assert_eq!(desc, "A test skill.");
+        assert!(!hidden);
     }
 
     #[test]
     fn test_parse_frontmatter_multiline_description() {
         let content =
             "---\nname: test\ndescription: First line\n  continued here\n  and here\n---\n";
-        let (name, desc) = parse_frontmatter(content).unwrap();
+        let (name, desc, hidden) = parse_frontmatter(content).unwrap();
         assert_eq!(name, "test");
         assert_eq!(desc, "First line continued here and here");
+        assert!(!hidden);
+    }
+
+    #[test]
+    fn test_parse_frontmatter_hidden_true() {
+        let content = "---\nname: stub\ndescription: A bootstrap stub.\nhidden: true\n---\n";
+        let (name, desc, hidden) = parse_frontmatter(content).unwrap();
+        assert_eq!(name, "stub");
+        assert_eq!(desc, "A bootstrap stub.");
+        assert!(hidden);
+    }
+
+    #[test]
+    fn test_parse_frontmatter_hidden_false() {
+        let content = "---\nname: visible\ndescription: Visible.\nhidden: false\n---\n";
+        let (_, _, hidden) = parse_frontmatter(content).unwrap();
+        assert!(!hidden);
     }
 
     #[test]