ci: add a gate for references/ links in skills

The path fix has no regression guard: nothing in CI resolves references/
links, so all 18 were broken while CI stayed green. validate-artifact-
paths.js is scoped to spec/plan/todo artifacts and says in its own header
that it is not a general markdown path linter.

Add validate-reference-links.js, which resolves every `references/*.md`
link in skills/*/SKILL.md against that skill's own directory. This accepts
both conventions in CLAUDE.md: shared checklists reached via
../../references/, and a skill's own colocated references/ directory.

Scope stays narrow on purpose. Skills legitimately name paths that do not
exist yet -- tasks/todo.md, PERF.md, docs/ideas/[idea-name].md -- and a
general markdown linter would fail the build on them. A test pins that.

Proven against the pre-fix tree: 18 error(s), exit 1, matching the 18
links fixed in the previous commit. After the fix: 0 error(s), exit 0.

7 unit tests cover the regression itself, colocated references/,
markdown-link syntax, a renamed target, multiple violations in one skill,
and the non-reference paths that must be ignored. Wired into the
validate-skills job, alongside the other skill-content checks.

Known limitation: fenced code blocks are not stripped, so a SKILL.md that
documents the anti-pattern inside a fence would be flagged. Nothing does
today. Sharing stripFencedCodeBlocks looks right once #444 lands.

Refs #468

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/test-plugin-install.yml

@@ -34,6 +34,12 @@ jobs:
       - name: Run skill evals (trigger + routing)
         run: node scripts/run-evals.js --min-rank1 80
 
+      - name: Validate references/ links in skills
+        run: node scripts/validate-reference-links.js
+
+      - name: Test reference-link validator
+        run: node --test scripts/validate-reference-links-test.js
+
   validate-commands:
     name: Validate command parity and description sync
     runs-on: ubuntu-latest