docs: add contributor guide and repo skill

skills/contributing/SKILL.md

@@ -0,0 +1,28 @@
+---
+name: contributing
+description: Contribute changes to the Feynman repository itself. Use when the task is to add features, fix bugs, update prompts or skills, change install or release behavior, improve docs, or prepare a focused PR against this repo.
+---
+
+# Contributing
+
+Read `CONTRIBUTING.md` first, then `AGENTS.md` for repo-level agent conventions.
+
+Use this skill when working on Feynman itself, especially for:
+
+- CLI or runtime changes in `src/`
+- prompt changes in `prompts/`
+- bundled skill changes in `skills/`
+- subagent behavior changes in `.feynman/agents/`
+- install, packaging, or release changes in `scripts/`, `README.md`, or website docs
+
+Minimum local checks before claiming the repo change is done:
+
+```bash
+npm test
+npm run typecheck
+npm run build
+```
+
+If the docs site changed, also validate `website/`.
+
+When changing release-sensitive behavior, verify that `.nvmrc`, package `engines`, runtime guards, and install docs stay aligned.