test: share one skill-structure lint across the four plugin suites

[gtapps]

Summary

The skill-structure lint existed in four copies and tests/test-utils.ts was byte-identical (md5 1ffd9c8f9149eb75ff6f86b886f5e4f2) in dev, fitness, and forge. Because the copies drifted independently, parser improvements never propagated: only feed's version folded YAML block scalars, so a description: >- in any dev/fitness/forge skill would have been read as the literal >- and failed lint as "description too short". This extracts the shared checks into one repo-root module and adopts feed's parser as the shared one.

Monorepo dev tooling only — never shipped to operators, which is why it may live at the repo root and cross plugin boundaries (same territory as tests/cross-plugin/). No CHANGELOG entries: root-scope and test-only changes aren't something an operator experiences.

Changes

• New tests/lib/skill-lint.ts — parseFrontmatter (dev's {raw, fields, body} shape + feed's block-scalar folding), makeReporter (moved verbatim from the deleted copies), and lintSkills(pluginRoot, skills). lintSkills is pure: it returns failure messages and never prints or exits, so both the console-reporter suites (dev/fitness/forge, which run each lint as its own bun process) and feed's bun test consume it without touching forge's shared-runner constraint.
• Four per-plugin lints shrunk to expectations + their own checks — dev keeps its ${CLAUDE_PLUGIN_ROOT}/scripts/ dangling-reference check, fitness its CLAUDE-APPEND trim guard, forge its PHP-floor assertion; feed's first loop now calls lintSkills and its local parser is gone.
• Three test-utils.ts copies deleted, with all five importers repointed.
• CI: tests/lib/** added to the path filters of test-dev, test-fitness, test-forge, test-feed, and test-cross-plugin; previously an edit to shared test code matched no filter and ran in zero suites. The cross-plugin job also runs tests/lib/.

Side effect worth noting: feed's per-skill checks previously asserted only name/description; routing through lintSkills also applies the gate-count and internal-link checks to its skills. That's an expansion of what CI enforces for that plugin, consistent with the other three.

Test plan

All run and green on the rebased branch:

• bash tests/run-all.sh in claude-code-dev-hermit (13 passed), claude-code-fitness-hermit (129 passed), laravel-forge-hermit (13 passed, incl. PHP suite) — exit 0 each
• bun test in feed-hermit — 54 passed
• bun test tests/lib/ tests/cross-plugin/ — 110 passed
• bunx tsc --noEmit from repo root (strict) — exit 0
• Mutation check: changing dev's dev-pr expectation from 5 gates to 4 fails with dev-pr: expected 4 Gate headers, found 5; reverted and green