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>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,29 @@
+<!--
+  Thanks for contributing! See CONTRIBUTING.md and GOVERNANCE.md.
+  A substantive PR needs a backing accepted issue or accepted RFC.
+  Maintainers: your internal process applies; the link requirement below
+  is for external contributions.
+-->
+
+## What & why
+
+<!-- One or two sentences: what this changes and why. -->
+
+## Backing issue / RFC
+
+<!-- Pick one. A substantive change needs (1) or (2). -->
+
+- [ ] Fixes an **accepted** issue: Closes #
+- [ ] Implements / is an **accepted** RFC: <link to docs/rfcs/NNNN-*.md>
+- [ ] **Trivial fast-lane** (typo / docs / dependency bump / comment / one-line CI) — no issue/RFC required
+
+## Checklist
+
+- [ ] Change is focused (one logical change)
+- [ ] Tests added/updated for behavior changes (or N/A)
+- [ ] Public docs updated if user-facing surface changed (or N/A)
+- [ ] Reviewed against [docs/dev/invariants.md](../blob/main/docs/dev/invariants.md) — no Hard Invariant weakened, no deny-list item hit (or justified)
+
+## Notes for reviewers
+
+<!-- Anything that helps review: tradeoffs, follow-ups, areas of risk. -->