docs: add CONTRIBUTING.md codifying the project philosophy (#80)

Codifies what has so far lived in PR closing comments: the universal-
template rule, merge/decline criteria with precedent links, the
feature-complete command bar, the empirical-verification standard,
and the fork path via /add-portal and the community forks discussion.

GitHub auto-surfaces CONTRIBUTING.md on new PRs and issues; README
gets a one-line pointer for browsers.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -309,6 +309,10 @@ The framework supports two distinct modes of job searching:
 
 To get the most from this, invest time during `/setup` in describing not just your experience, but what energized you, what drained you, and what you'd want more of. This context directly shapes how the system evaluates fit and which roles it surfaces during `/scrape`.
 
+## Contributing
+
+Thinking about a PR? Read [CONTRIBUTING.md](CONTRIBUTING.md) first - it explains what gets merged, what lives in forks, and why.
+
 ## Acknowledgements
 
 - [Mikkel Krogholm](https://github.com/mikkelkrogsholm) ([skills repo](https://github.com/mikkelkrogsholm/skills)) for the job search CLI skills