chore(guards): add durable markers so fixes aren't silently dropped

Introduce AGENTS.md (root, primary agent instruction file) documenting six
hard-won fixes with explicit DO NOT / WHY, plus executable enforcement so a
future AI cannot delete or reintroduce them:

1. package-lock.json must be generated with npm 10 (Docker's npm 10.9.8).
   npm 11 drops the top-level @emnapi/core + @emnapi/runtime entries npm 10
   needs, breaking the tag-triggered Docker build at `npm ci` (happened on
   v1.0.14). Add scripts/verify-lockfile-npm10.mjs + .npmrc + a Dockerfile
   fail-fast check + a CI step + tests/unit/lockfile-npm10-guard.test.js.
   Also re-fix the lockfile itself (regenerated with npm 10.9.8).
2. Tests must never write to the real ~/.9router DB (isolateDataDir).
3. Hidden providers must not leak into Usage (usageProviders !p.hidden).
4. codebuddy-intl connection test + OAuth identity.
5. Fork-only features that must survive upstream syncs.
6. Upstream sync procedure.

Each marker cross-references AGENTS.md and the covering test. CLAUDE.md now
points to AGENTS.md at the top. Verified: build ok, guard script passes,
full suite leaves the real DB count unchanged (38), 0 new regressions.

.npmrc

@@ -0,0 +1,17 @@
+# MIBP fork — npm behavior pin.
+#
+# DO NOT DELETE. See AGENTS.md §1.
+#
+# The Docker image (node:22-alpine, pinned by digest) ships npm 10.9.8 and runs
+# `npm ci` against the committed package-lock.json. Regenerating the lockfile
+# with npm 11+ drops the top-level @emnapi/core + @emnapi/runtime entries npm 10
+# needs, which breaks the tag-triggered Docker build ("Build and Push Docker
+# Image") at `npm ci`.
+#
+# Regenerate the lockfile with npm 10 only:
+#   npx -y npm@10.9.8 install --package-lock-only
+#   node scripts/verify-lockfile-npm10.mjs
+#
+# Keep install output quiet; never let audit/fund noise hide a lockfile error.
+audit=false
+fund=false