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/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,14 @@
+## Type of Change
+
+- [ ] **Skill** - adds a new skill in `.claude/skills/`
+- [ ] **Fix** - bug fix or security fix to source code
+- [ ] **Simplification** - reduces or simplifies source code
+
+## Description
+
+
+## For Skills
+
+- [ ] I have not made any changes to source code
+- [ ] My skill contains instructions for Claude to follow (not pre-built code)
+- [ ] I tested this skill on a fresh clone