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.

tests/setup/isolateDataDir.js

@@ -4,6 +4,9 @@
 // they append test rows ("zed-live-*@example.com", "guard-*@example.com",
 // "Account N") straight into the live DB.
 //
+// GUARD — DO NOT DELETE this file or remove it from vitest.config.js setupFiles.
+// See AGENTS.md §2. Covered by tests/unit/test-data-dir-isolation.test.js.
+//
 // DATA_DIR must be set before src/lib/dataDir.js is imported (it reads the env
 // at module-eval time), which is exactly what a vitest setupFile guarantees.
 //