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>

CONTRIBUTING.md

@@ -0,0 +1,23 @@
+# Contributing
+
+## Source Code Changes
+
+**Accepted:** Bug fixes, security fixes, simplifications, reducing code.
+
+**Not accepted:** Features, capabilities, compatibility, enhancements. These should be skills.
+
+## Skills
+
+A [skill](https://code.claude.com/docs/en/skills) is a markdown file in `.claude/skills/` that teaches Claude Code how to transform a NanoClaw installation.
+
+A PR that contributes a skill should not modify any source files.
+
+Your skill should contain the **instructions** Claude follows to add the feature—not pre-built code. See `/convert-to-docker` for a good example.
+
+### Why?
+
+Every user should have clean and minimal code that does exactly what they need. Skills let users selectively add features to their fork without inheriting code for features they don't want.
+
+### Testing
+
+Test your skill by running it on a fresh clone before submitting.