Auto-check CONTRIBUTING.md compliance before creating PRs

[costajohnt]

Problem

During the nickel-lang/nickel#2529 PR workflow, the contribution guidelines check was not automatic. The user had to explicitly ask "did you read the contribution guidelines?" before we verified compliance.

The CONTRIBUTING.md for nickel required:

• Updating the user manual for user-facing changes
• Writing extensive tests
• Reporting benchmarks if perf-impacting

We did comply, but only caught the manual update requirement because the code reviewer flagged it, not because the workflow checked proactively.

Proposal

Before creating a PR (in the draft-first workflow), automatically:

1. Read CONTRIBUTING.md / HACKING.md / similar files
2. Extract actionable requirements (docs, tests, changelog, etc.)
3. Verify each requirement is met
4. Report any gaps before the PR is created

This should be a standard step in the draft-first workflow, not something the user has to remember to ask for.