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>

scripts/check-agents-md.sh

@@ -34,10 +34,15 @@ PY
 canonical=()
 while IFS= read -r line; do
   canonical+=("$line")
-done < <(find docs -type f -name '*.md' ! -path 'docs/releases/*' ! -path 'docs/internal/*' | sort)
+done < <(find docs -type f -name '*.md' ! -path 'docs/releases/*' ! -path 'docs/internal/*' ! -path 'docs/rfcs/*' | sort)
 if [[ -d docs/releases ]]; then
   canonical+=("docs/releases/")
 fi
+# RFCs are a growing collection (like releases): represent the directory, not
+# every per-RFC file. The dir must be linked from an audience index.
+if [[ -d docs/rfcs ]]; then
+  canonical+=("docs/rfcs/")
+fi
 
 linked=()
 for index_file in "${index_files[@]}"; do