fix(skills): validate bundled SKILL.md front-matter in CI (fixes #2443) (#2457)

* fix(skills): validate bundled SKILL.md front-matter in CI (fixes #2443)

Adds a parametrized backend test that runs `_validate_skill_frontmatter`
against every bundled SKILL.md under `skills/public/`, so a broken
front-matter fails CI with a per-skill error message instead of
surfacing as a runtime gateway-load warning.

The new test caught two pre-existing breakages on `main` and fixes them:

* `bootstrap/SKILL.md`: the unquoted description had a second `:` mid-line
  ("Also trigger for updates: ..."), which YAML parses as a nested mapping
  ("mapping values are not allowed here"). Rewrites the description as a
  folded scalar (`>-`), which preserves the original wording (including the
  embedded colon, double quotes, and apostrophes) without further escaping.
  This complements PR #2436 (single-file colon→hyphen patch) with a more
  general convention that survives future edits.

* `chart-visualization/SKILL.md`: used `dependency:` which is not in
  `ALLOWED_FRONTMATTER_PROPERTIES`. Renamed to `compatibility:`, the
  documented field for "Required tools, dependencies" per skill-creator.
  No code reads `dependency` (verified by grep across backend/).

* Apply suggestions from code review

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Fix the lint error

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

skills/public/chart-visualization/SKILL.md

@@ -1,7 +1,7 @@
 ---
 name: chart-visualization
 description: This skill should be used when the user wants to visualize data. It intelligently selects the most suitable chart type from 26 available options, extracts parameters based on detailed specifications, and generates a chart image using a JavaScript script.
-dependency:
+compatibility:
   nodejs: ">=18.0.0"
 ---