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.

skill-data/core/templates/capture-workflow.sh

@@ -0,0 +1,69 @@
+#!/bin/bash
+# Template: Content Capture Workflow
+# Purpose: Extract content from web pages (text, screenshots, PDF)
+# Usage: ./capture-workflow.sh <url> [output-dir]
+#
+# Outputs:
+#   - page-full.png: Full page screenshot
+#   - page-structure.txt: Page element structure with refs
+#   - page-text.txt: All text content
+#   - page.pdf: PDF version
+#
+# Optional: Load auth state for protected pages
+
+set -euo pipefail
+
+TARGET_URL="${1:?Usage: $0 <url> [output-dir]}"
+OUTPUT_DIR="${2:-.}"
+
+echo "Capturing: $TARGET_URL"
+mkdir -p "$OUTPUT_DIR"
+
+# Optional: Load authentication state
+# if [[ -f "./auth-state.json" ]]; then
+#     echo "Loading authentication state..."
+#     agent-browser state load "./auth-state.json"
+# fi
+
+# Navigate to target
+agent-browser open "$TARGET_URL"
+agent-browser wait --load networkidle
+
+# Get metadata
+TITLE=$(agent-browser get title)
+URL=$(agent-browser get url)
+echo "Title: $TITLE"
+echo "URL: $URL"
+
+# Capture full page screenshot
+agent-browser screenshot --full "$OUTPUT_DIR/page-full.png"
+echo "Saved: $OUTPUT_DIR/page-full.png"
+
+# Get page structure with refs
+agent-browser snapshot -i > "$OUTPUT_DIR/page-structure.txt"
+echo "Saved: $OUTPUT_DIR/page-structure.txt"
+
+# Extract all text content
+agent-browser get text body > "$OUTPUT_DIR/page-text.txt"
+echo "Saved: $OUTPUT_DIR/page-text.txt"
+
+# Save as PDF
+agent-browser pdf "$OUTPUT_DIR/page.pdf"
+echo "Saved: $OUTPUT_DIR/page.pdf"
+
+# Optional: Extract specific elements using refs from structure
+# agent-browser get text @e5 > "$OUTPUT_DIR/main-content.txt"
+
+# Optional: Handle infinite scroll pages
+# for i in {1..5}; do
+#     agent-browser scroll down 1000
+#     agent-browser wait 1000
+# done
+# agent-browser screenshot --full "$OUTPUT_DIR/page-scrolled.png"
+
+# Cleanup
+agent-browser close
+
+echo ""
+echo "Capture complete:"
+ls -la "$OUTPUT_DIR"