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/DISCUSSION_TEMPLATE/rfc.yml

@@ -0,0 +1,34 @@
+labels: ["rfc"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Use this to **incubate an RFC** — socialize a design and reach rough
+        consensus before writing the formal document. When it's ready, graduate
+        it into a pull request that adds `docs/rfcs/NNNN-title.md`
+        (see [docs/rfcs/README.md](../blob/main/docs/rfcs/README.md)); a
+        maintainer merging that PR is acceptance.
+
+        For a plain feature request or open-ended idea, use the **Ideas**
+        category instead. For bugs, open an [Issue](../../issues/new/choose).
+  - type: textarea
+    id: problem
+    attributes:
+      label: Problem / motivation
+      description: What needs solving, and why is it worth the long-run cost?
+    validations:
+      required: true
+  - type: textarea
+    id: sketch
+    attributes:
+      label: Proposed direction (sketch)
+      description: A rough shape of the design. Detail comes later in the RFC document.
+    validations:
+      required: true
+  - type: textarea
+    id: invariants
+    attributes:
+      label: Invariants touched
+      description: Which items in docs/dev/invariants.md does this affect or risk? Any deny-list brush?
+    validations:
+      required: false