governance: external contribution model (issues/discussions/RFCs/PRs) (#143)

Formalize the public contribution surface. Maintainers keep a separate internal
process and are exempt from the intake gates; everyone stays bound by review,
CODEOWNERS, and branch protection.

Model:
- Issues = problem reports only (bug form + config.yml redirects ideas to
  Discussions and disables blank issues).
- Discussions = ideas + RFC incubation.
- RFCs = anyone (incl. external) authors docs/rfcs/NNNN-*.md; a maintainer
  merging it is acceptance. Distinct from the maintainer-internal
  docs/dev/rfc-00N-* track.
- PRs = link an `accepted` issue or accepted RFC, or use the trivial fast-lane
  (typos/docs/deps). Enforced softly to start (template + review).

Adds GOVERNANCE.md, rewrites CONTRIBUTING.md, adds docs/rfcs/ (README +
template), .github issue/PR/discussion templates. Wires docs/rfcs/ into the
doc-link checker (excluded like releases; linked from docs/dev/index.md).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/dev/index.md

@@ -51,6 +51,18 @@ constraints. User-facing behavior should still be documented through
 | Install and deployment packaging | [install.md](../user/install.md), [deployment.md](../user/deployment.md) |
 | Release history | [releases/](../releases/) |
 
+## Contribution & Governance
+
+| Area | Read |
+|---|---|
+| How to contribute (external) | [CONTRIBUTING.md](../../CONTRIBUTING.md) |
+| Governance model, roles, decision authority | [GOVERNANCE.md](../../GOVERNANCE.md) |
+| Public contribution RFC track | [rfcs/](../rfcs/) |
+
+The `docs/rfcs/` track is the **public, externally-authorable** RFC process. The
+maintainer/internal RFCs below (`rfc-00N-*.md`) are a separate, team-owned
+track; don't conflate the two.
+
 ## Active Implementation Plans
 
 Working documents for in-flight feature work. Removed when the work lands.