Add marketplace.json validation CI workflow (#347)

* Add CI workflow to validate marketplace.json on PRs

Add a GitHub Actions workflow that validates marketplace.json is
well-formed JSON with a plugins array whenever PRs modify it. Includes:
- validate-marketplace.ts: Bun script that parses and validates the JSON
- validate-marketplace.yml: GH Actions workflow triggered on PR changes
- test-marketplace-check.js: Unit tests for the validation logic

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Strengthen marketplace validator and remove orphaned test file

- validate-marketplace.ts: check duplicate names and required fields
  (name, description, source) per entry, not just valid JSON
- remove .github/workflows/test-marketplace-check.js: tested a
  checkMarketplaceViolations function that doesn't exist in the PR,
  and was in workflows/ instead of scripts/

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Tobin South <tobin.south@gmail.com>

.github/workflows/validate-marketplace.yml

@@ -0,0 +1,17 @@
+name: Validate Marketplace JSON
+
+on:
+  pull_request:
+    paths:
+      - '.claude-plugin/marketplace.json'
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: oven-sh/setup-bun@v2
+
+      - name: Validate marketplace.json
+        run: bun .github/scripts/validate-marketplace.ts .claude-plugin/marketplace.json