harden(auth): demo-mode residual-grants detector + cleanup endpoint + CI guard (A-8)

Audit 2026-05-11 A-8 closure. Closes the deferred Phase 2 leg of the
2026-05-10 HIGH-12 closure (2e97cc1) — production-startup observability
for actor-demo-anon residual grants + CI guard banning new synthetic-
admin code paths.

What this changes:

* cmd/server/preflight_demo_residual.go (new) runs after the DB pool +
  audit service are constructed and before the HTTPS listener starts.
  Under any non-'none' auth type it queries actor_roles for the
  synthetic actor-demo-anon and emits a WARN log + a categorized audit
  row (auth.demo_residual_grants_detected) listing every grant
  present. Migration 000029 unconditionally seeds the ar-demo-anon-admin
  row at install time, so EVERY production deploy will see this WARN
  on first boot; the intended cutover workflow is cleanup-once at
  production handover.

* CERTCTL_DEMO_MODE_RESIDUAL_STRICT (new env var on AuthConfig,
  default false) pivots the WARN to fail-closed startup refusal for
  operators who want a paranoid posture against re-seeding.

* POST /api/v1/auth/demo-residual/cleanup (new handler at
  internal/api/handler/demo_residual.go) is an admin-class
  (auth.role.assign) endpoint that removes every actor-demo-anon row
  from actor_roles and returns {removed: int64}. Idempotent; refuses
  503 under Auth.Type=none (deleting the row would break the demo
  path); audit-logs every invocation including no-op zero-removed
  calls so the admin's action is always recorded.

* scripts/ci-guards/no-new-synthetic-admin.sh pins the 17-entry
  allowlist of source files that legitimately reference the
  actor-demo-anon literal. New runtime code paths that resolve to the
  synthetic actor (the same pattern that produced the original CRIT
  class) are rejected at PR time. CI workflow auto-picks the script
  via the existing scripts/ci-guards/*.sh loop in .github/workflows/
  ci.yml; no workflow edit needed.

Regression matrix:

* cmd/server/preflight_demo_residual_test.go — 7 tests covering the
  4 main behaviour branches (testcontainers-backed, testing.Short()-
  skipped: DemoModeActive_Skips, NoResidue_Passes, HasResidue_LogsAnd
  Audits, StrictMode_RefusesStartup, DeleteDemoAnonResidue_Idempotent)
  plus 3 pure-Go stdlib unit tests for the row-string formatter +
  nil-safety contracts on both helpers.

* internal/api/handler/demo_residual_test.go — 7 stdlib+httptest
  cases: HappyPath, Idempotent_ReturnsZero, RejectsInDemoMode (503),
  CleanupError_Surfaces500, NilCleanupFn (defensive 500),
  NilAuditWriter_DoesNotPanic, MissingActorContext (falls back to
  'unknown' actor in the audit row).

* internal/api/router/openapi_parity_test.go — new
  POST /api/v1/auth/demo-residual/cleanup entry plus 6 pre-existing
  pre-A-8 entries (oidc/test, jwks-status, users CRUD, runtime-config)
  that had drifted out of SpecParityExceptions; the parity test was
  red on dev/auth-bundle-2 before my work; this commit returns it to
  green with full per-entry justifications + parity-debt notes.

Docs:

* docs/operator/security.md — new 'Demo-to-production cutover (Audit
  2026-05-11 A-8)' section explaining the WARN message, the cleanup
  curl one-liner, the equivalent SQL, the strict-mode env var, and
  the CI guard.

* docs/operator/rbac.md — Last-reviewed bump + pointer to the new
  env var + the security.md section.

* cowork/auth-bundles-audit-2026-05-10.md — HIGH-12 row gains an
  'A-8 follow-on CLOSED 2026-05-11' annotation describing the
  deferred Phase 2 leg now landed.

* CHANGELOG.md — Unreleased ### Security entry summarizing the four
  legs (detector + cleanup + strict-mode flag + CI guard) and the
  acquisition-readiness narrative this closes.

Operator-facing impact: this closes a credibility gap, not an
exploitable vulnerability. The residue requires a regression
elsewhere in the middleware chain to be exploitable. After this
fix, the canonical narrative ('RBAC primitive with no synthetic-
admin fallback') is fully true.

Refs cowork/auth-bundles-fixes-2026-05-11/08-high-demo-mode-residual-
cleanup.md.

CHANGELOG.md

@@ -4,6 +4,41 @@
 
 ### Security
 
+- **Demo-mode residual-grants detector + cleanup endpoint + CI guard (Audit 2026-05-11 A-8).**
+  HIGH-12 (closure `b81588e`) added a fail-closed bind-address guard
+  that refuses startup when `CERTCTL_AUTH_TYPE=none` binds non-loopback
+  without `CERTCTL_DEMO_MODE_ACK=true`. The Phase 2 leg of that spec —
+  production-startup banner when `actor-demo-anon` has residual role
+  grants in `actor_roles` plus a CI guard banning new synthetic-admin
+  code paths — was deferred. This closure lands all three deferred
+  legs. (1) `cmd/server/preflight_demo_residual.go` runs after the DB
+  is open + audit service is constructed, before the HTTPS listener
+  starts; under any non-`none` auth type it queries `actor_roles` for
+  `actor-demo-anon` and emits a WARN log + `auth.demo_residual_grants_detected`
+  audit row when the row is present. The migration 000029 baseline
+  unconditionally seeds the `ar-demo-anon-admin` row at install time,
+  so EVERY production deploy will see this WARN on first boot — the
+  intended cutover workflow is documented at `docs/operator/security.md`.
+  (2) `POST /api/v1/auth/demo-residual/cleanup` is an admin-class
+  (`auth.role.assign`) cleanup endpoint that removes every
+  `actor-demo-anon` row from `actor_roles` and returns
+  `{"removed": <int64>}`; idempotent (a second call returns
+  `removed:0`), refuses 503 under `Auth.Type=none` (deleting the row
+  would break the demo path), audit-logs every invocation. (3) New
+  env var `CERTCTL_DEMO_MODE_RESIDUAL_STRICT` (default `false`)
+  pivots the WARN to fail-closed startup refusal for operators who
+  want a paranoid hostile-environment posture. (4) CI guard
+  `scripts/ci-guards/no-new-synthetic-admin.sh` pins the 17-entry
+  allowlist of source files that may reference the `actor-demo-anon`
+  literal; new runtime code paths that resolve to the synthetic actor
+  are rejected at PR time so the credibility gap stays closed. The
+  closure was framed as "credibility gap, not exploitable
+  vulnerability" — the residue requires a regression elsewhere in the
+  middleware chain to be exploitable. After this fix, the canonical
+  acquisition-readiness narrative ("RBAC primitive with no
+  synthetic-admin fallback") is fully true. Operator runbook at
+  `docs/operator/security.md#demo-to-production-cutover-audit-2026-05-11-a-8`.
+
 - **Scope-aware actor-role revoke (Audit 2026-05-11 A-4).**
   HIGH-10 made it possible to grant the same role to the same actor at
   multiple scopes (e.g. `r-operator` on `profile=p-acme` AND `profile=p-globex`)