RFC: AI contribution conventions — PRs, commits, comments, attribution

## Why
Multiple maintainers using multiple AI tools means inconsistent PR shapes, commit styles, comment density, and attribution if left undefined. Worth landing a short RFC/ADR that defines, tool-agnostically, what AI-assisted contributions should look like — for *any* contributor using *any* AI tool, not Claude- or Copilot-specific.

**Framed as a discussion/RFC, not a deliverable.** The conventions are opinionated and need maintainer agreement before they land as enforced rules.

## Scope to cover
- **Attribution:** when and how to credit AI involvement. `Co-Authored-By` lines are already used for Claude-driven commits — should Copilot and others follow the same shape? Should the AI's role (drafted / reviewed / executed verbatim) be distinguishable, or is "any AI involvement = one Co-Authored-By line" enough?
- **PR description style:** how much "explain my reasoning" detail an AI-assisted PR should include. Real tension between useful reviewer context and a wall of AI-generated text that obscures the actual change.
- **Comments in code:** AI tools love adding comments. CLAUDE.md already says "default to writing no comments". Promote to a repo-wide rule for humans too, or keep AI-specific?
- **Review expectations:** do AI-authored PRs go through the same review path as human ones, a faster one (rubber-stamp if CI green?), or a stricter one (assume hallucinations until proven otherwise)?
- **What's NOT acceptable:** speculative refactors, doc-only PRs that add no value, hallucinated API references, changelog entries not grounded in the diff, etc.
- **Commit hygiene:** AI tools produce many small commits. Squash-on-merge default, or guidance on commit granularity?

## Action
- [ ] Open as a GitHub Discussion (or keep this issue as the thread) — gather maintainer input
- [ ] Draft conventions as an ADR under `docs/adr/`
- [ ] Land the outcome in `CONTRIBUTING.md` and/or `AGENTS.md`
- [ ] Decide if any of this is machine-enforced (commit-msg hook, PR-template required fields) or stays social

## Coordinate with
- AGENTS.md adoption issue — outcome lives there
- Data-handling policy issue — adjacent concern

## Done when
- ADR drafted and merged
- `CONTRIBUTING.md` / `AGENTS.md` reference the conventions
- (Optional) machine-enforced bits wired into hooks or PR templates