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.

docs/rfcs/0000-template.md

@@ -0,0 +1,54 @@
+# RFC NNNN: <title>
+
+| | |
+|---|---|
+| **Status** | Proposed |
+| **Author(s)** | <your name / handle> |
+| **Discussion** | <link to the originating Discussion, if any> |
+| **Implementation** | <issue/PR links, filled in as work lands> |
+
+> Status is maintained by maintainers: `Proposed` while the PR is open,
+> `Accepted` on merge, `Declined` on close, `Superseded by NNNN` later.
+
+## Summary
+
+One paragraph: what this changes, in plain terms.
+
+## Motivation
+
+What problem does this solve, and why is it worth the ongoing cost? Tie it to a
+concrete need (a Discussion, a recurring issue, a user request). Per the
+project's first principle, argue the *long-run liability*, not just the
+short-term convenience.
+
+## Guide-level explanation
+
+Explain the change as you'd teach it to a user or contributor: new commands,
+syntax, API shapes, behavior. Examples first.
+
+## Reference-level design
+
+The precise design: data structures, IR/AST/planner changes, storage/format
+impact, migration path, error behavior. Enough that a reviewer can find the
+holes.
+
+## Invariants & deny-list check
+
+Which Hard Invariants in [../dev/invariants.md](../dev/invariants.md) does this
+touch? Does it brush against any deny-list item — and if so, why is this the
+justified exception? State explicitly that no invariant is weakened, or which
+Known Gap moves.
+
+## Drawbacks & alternatives
+
+What does this cost, what did you reject, and why. "Do nothing" is a valid
+alternative to weigh.
+
+## Reversibility
+
+Is this reversible? On-disk/wire/format and substrate choices are near-permanent
+and demand more evidence; a CLI flag or doc is cheap to undo. Say which this is.
+
+## Unresolved questions
+
+What's deliberately left open for review to settle.

docs/rfcs/README.md

@@ -0,0 +1,66 @@
+# RFCs
+
+Substantial changes to OmniGraph — new user-facing surface, format or protocol
+changes, anything irreversible or cross-cutting — go through a lightweight RFC
+so the design is agreed *as reviewable code* before implementation starts. This
+is the public RFC track, open to **anyone, including external contributors**.
+
+This complements the always-on review bar in
+[../dev/invariants.md](../dev/invariants.md): the invariants say *what every
+change must respect*; an RFC says *why this particular change is worth making and
+how*.
+
+> **Two tracks, don't conflate them.** This `docs/rfcs/` directory is the
+> **public contribution** track (anyone authors; maintainers accept). The
+> maintainer-internal RFCs under `docs/dev/rfc-00N-*.md` are a separate,
+> team-owned track for in-flight internal work. If you're an outside
+> contributor, you're in the right place here.
+
+## When you need one
+
+- **RFC required:** new query/schema/CLI/HTTP surface; on-disk or wire-format
+  changes; a new substrate dependency; anything the deny-list in
+  [../dev/invariants.md](../dev/invariants.md) flags; anything irreversible
+  ("reversibility shapes evidence demand").
+- **RFC not required:** bug fixes for an `accepted` issue, and the trivial
+  fast-lane (typos, docs, deps) — see [../../CONTRIBUTING.md](../../CONTRIBUTING.md).
+
+If you're unsure, start a [Discussion](../../../discussions); a maintainer will
+tell you whether it needs an RFC.
+
+## Lifecycle
+
+```
+Discussion (incubate, get rough consensus)
+      │ graduate
+      ▼
+RFC pull request  →  adds docs/rfcs/NNNN-title.md  (Status: Proposed)
+      │
+maintainer review  ──▶  changes requested / declined (PR closed, with rationale)
+      │
+      ▼
+merged  ==  Accepted   (the merged file is the durable decision record)
+      │
+      ▼
+Implementation PR(s)  reference the accepted RFC
+```
+
+- **Author:** anyone. **Acceptance:** a maintainer decision, performed by
+  merging the RFC PR. Declining is closing it with rationale.
+- The merged RFC *is* the accepted record — there is no separate sign-off step.
+- Later reversals don't edit history: supersede with a new RFC that links back
+  and flip the old one's `Status` to `Superseded`.
+
+## Numbering & naming
+
+- File: `docs/rfcs/NNNN-kebab-title.md`, where `NNNN` is the next free
+  zero-padded integer (`0001`, `0002`, …). `0000-template.md` is reserved.
+- Pick the number when you open the PR; if it collides with another in-flight
+  RFC, the second to merge bumps theirs.
+
+## Status values
+
+`Proposed` (open PR) · `Accepted` (merged) · `Declined` (closed) ·
+`Superseded by NNNN` · `Implemented` (set once the work lands, optional).
+
+Copy [0000-template.md](0000-template.md) to start.