ci(governance): migrate R5a to .github/canonical-references/ — drop local workflow (#174)

## Summary

Three coupled changes following
[hyperpolymath/standards#330](hyperpolymath/standards#330)
(generic config-driven R5 in the reusable):

1. **Add `.github/canonical-references/prover-counts.yml`** — the
echidna R5a rule (bare prover counts → `docs/PROVER_COUNT.md`) declared
in the upstream config schema. Same patterns, same anchoring, same
scope.

2. **Bump `governance.yml` SHA pin** `bf406d6f…` → `3f34549c…`
(standards#330 merge SHA). The reusable's new R5 generic step now
consumes the config file above; R5b (estate-wide version-string drift)
continues to apply in parallel as before.

3. **Delete `.github/workflows/governance-doc-drift.yml`** — the local
R5a workflow is now obsolete. The upstream R5 generic step provides the
same coverage with a tighter feedback loop (rule changes are config-only
PRs; no workflow editing required).

## Behaviour preservation

- ✅ Pattern set identical (7 alternations covering the drift class
catalogued in #171)
- ✅ Scope identical (14 load-bearing top-level docs)
- ✅ Anchor preserved — `(^|[[:space:]])` so `Tier-1`, `v1.2.3`, `port
8081` don't false-positive
- ✅ Canonical-pointer error routes the same way (`docs/PROVER_COUNT.md`)

## Test plan

- [x] Local re-run of the upstream R5 logic against current main + the
new config: **0 hits**
- [x] `.github/workflows/governance-doc-drift.yml` deletion is clean (no
other workflow refers to it)
- [ ] CI on this PR — `governance / Security policy checks` (now
containing R5 generic + R5b) green

Refs hyperpolymath/standards#330, refs #171.

Author: hyperpolymath

.github/canonical-references/prover-counts.yml

@@ -0,0 +1,50 @@
+# SPDX-License-Identifier: MPL-2.0
+# SPDX-FileCopyrightText: 2025-2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+#
+# R5 canonical-reference rule: bare prover counts in load-bearing
+# top-level docs must defer to docs/PROVER_COUNT.md (the canonical
+# tier table).
+#
+# Consumed by the `Canonical-reference drift (R5 generic)` step in
+# hyperpolymath/standards's governance-reusable.yml — see
+# hyperpolymath/standards#330. Replaces the previous local-only
+# .github/workflows/governance-doc-drift.yml (deleted in the same
+# commit).
+#
+# Rule format documented in the upstream step comment.
+
+id: prover-counts
+description: |
+  Bare prover counts (variants vs impl files vs advertised vs core)
+  in narrative-carrying top-level docs must defer to
+  docs/PROVER_COUNT.md so they cannot drift. Patterns are anchored
+  with (^|[[:space:]]) so neighbours like Tier-1, v1.2.3, port 8081
+  don't false-positive.
+
+patterns:
+  - '(^|[[:space:]])[0-9]+[[:space:]]+prover[[:space:]]+backends?\b'
+  - '(^|[[:space:]])[0-9]+[[:space:]]*-[[:space:]]*prover[[:space:]]+core\b'
+  - '(^|[[:space:]])[0-9]+[[:space:]]+core[[:space:]]+(backends?|provers?)\b'
+  - '(^|[[:space:]])[0-9]+[[:space:]]+(backends?|provers?)[[:space:]]+(operational|advertised|implemented|available|across|total)\b'
+  - '(^|[[:space:]])[0-9]+[[:space:]]+ProverKind[[:space:]]+variants?\b'
+  - '(^|[[:space:]])[0-9]+[[:space:]]+backend[[:space:]]+implementation[[:space:]]+files?\b'
+  - '(^|[[:space:]])[0-9]+/[0-9]+[[:space:]]+(backends?|provers?)\b'
+
+canonical_pointer: docs/PROVER_COUNT.md
+
+scope:
+  include:
+    - README.md
+    - README.adoc
+    - EXPLAINME.adoc
+    - CLAUDE.md
+    - CONTRIBUTING.md
+    - CONTRIBUTING.adoc
+    - QUICKSTART-USER.adoc
+    - QUICKSTART-DEV.adoc
+    - QUICKSTART-MAINTAINER.adoc
+    - RSR_COMPLIANCE.adoc
+    - SECURITY.md
+    - MAINTAINERS.adoc
+    - AUTHORS.md
+    - CODE_OF_CONDUCT.md

.github/workflows/governance-doc-drift.yml

@@ -31,4 +31,4 @@ permissions:
 
 jobs:
   governance:
-    uses: hyperpolymath/standards/.github/workflows/governance-reusable.yml@bf406d6f95c5212c34c30f6629be93d08c076339 # main 2026-06-01 (R5b version-string drift — standards#329)
+    uses: hyperpolymath/standards/.github/workflows/governance-reusable.yml@3f34549c03274ec7a74683068f03a492b0fa805f # main 2026-06-01 (R5 generic — standards#330; consumes .github/canonical-references/)