chore: retire completed-migration scaffolding, wire orphaned boundary check (#4719)

The ecstore/global-state migrations are done (backlog#815, #939, #1052 all
closed). Review of every migration-era test/gate measure found three things
actually retirable or broken — everything else is a live anti-regression
guard and stays.

Remove:
- scripts/check_metrics_migration_refs.sh — guards a migration that
  finished: rustfs_metrics:: has zero hits, the metrics crate no longer
  exists, and the script was never wired into CI or make (only reference
  was one line in config-model-boundary-adr.md, also removed).
- crates/obs init_metrics_collectors — the "backward-compatible alias kept
  during migration" the removed script was guarding. Zero callers; pure
  delegate to init_metrics_runtime.

Archive (docs/superpowers/plans/, continuing the 2026-07 convention,
with the standard archived banner):
- startup-timeline.md, scheduler-baseline.md,
  profiling-numa-capability-inventory.md,
  kms-development-defaults-inventory.md — one-shot snapshots whose only
  consumer is the already-archived migration-progress ledger (their
  same-dir links there start resolving again after the move); zero script
  pins; fed the closed backlog#660/#665 architecture-review ledger.
  Fixed the one outbound link (startup-timeline -> readiness-matrix) that
  the move would have broken — check_doc_paths.sh deliberately does not
  scan plans/, so nothing else would have caught it.

Wire (found orphaned by the same review):
- scripts/check_extension_schema_boundaries.sh guards a live contract
  crate but was never invoked anywhere. Add lint-fmt.mak target, include
  in pre-commit/pre-pr/dev-check, add ci.yml Quick Checks step (job
  already installs ripgrep), sync the CONTRIBUTING.md enumerated list,
  and harden the script against a silently-passing rg probe when src/
  is missing.

Keep (verified live, documented so the next cleanup pass does not repeat
this analysis):
- scripts/check_architecture_migration_rules.sh — added a header stating
  it is a permanent boundary guard, not retirable migration scaffolding;
  'migration' in the name is historical.
- check_migration_gate_count.sh + floor, delete-marker e2e proof, all
  pinned docs, compat-cleanup-register sync, remaining inventories
  (referenced by live docs).

Verification: all 7 guard scripts pass, actionlint clean,
cargo check --workspace (excl e2e) clean, cargo fmt --check clean.
Adversarially reviewed by two independent skeptic passes; their 7
findings (alias left behind, broken outbound link, missing banners,
wrong backlog attribution, CONTRIBUTING drift, rg exit-2 hole, missing
header rationale) are all folded in.

.github/workflows/ci.yml

@@ -128,6 +128,9 @@ jobs:
       - name: Check tokio io-uring feature guard
         run: ./scripts/check_no_tokio_io_uring.sh
 
+      - name: Check extension schema boundaries
+        run: ./scripts/check_extension_schema_boundaries.sh
+
   test-and-lint:
     name: Test and Lint
     if: github.event_name != 'pull_request' || github.event.action != 'closed'