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.

AGENTS.md

@@ -19,7 +19,7 @@ When adding or changing user-facing features (new flags, commands, behaviors, en
 
 1. `cli/src/output.rs` — `--help` output (flags list, examples, environment variables)
 2. `README.md` — Options table, relevant feature sections, examples
-3. `skills/agent-browser/SKILL.md` — so AI agents know about the feature
+3. `skill-data/core/SKILL.md` (and its `references/`) — so AI agents know about the feature when they load the core skill. Edit `skill-data/core/SKILL.md` for overview/workflow changes; edit `skill-data/core/references/*.md` for detailed reference content. Do **not** put feature content in `skills/agent-browser/SKILL.md` — that file is an intentionally thin discovery stub for `npx skills add` and exists only to redirect agents to `agent-browser skills get core`.
 4. `docs/src/app/` — the Next.js docs site (MDX pages)
 5. Inline doc comments in the relevant source files