Add contribution guidelines and PR checks for skills-only model

- CONTRIBUTING.md: Explain accepted source changes vs skills
- PR template: Checkboxes for contribution type
- CI workflow: Block PRs that add skills while modifying source
- CODEOWNERS: Require maintainer review for source changes

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/CODEOWNERS

@@ -0,0 +1,8 @@
+# Core code - maintainer only
+/src/ @gavrielc
+/container/ @gavrielc
+/package.json @gavrielc
+/package-lock.json @gavrielc
+
+# Skills - open to contributors
+/.claude/skills/