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.

Dockerfile

@@ -1,6 +1,9 @@
 # syntax=docker/dockerfile:1.7
 # Pinned by digest so a base-image refresh cannot silently bump npm and break
 # `npm ci` against the committed lockfile (see v1.0.9 npm ci EUSAGE failure).
+# DO NOT unpin. This image ships npm 10.9.8 — the lockfile MUST be regenerated
+# with npm 10 (`npx -y npm@10.9.8 install --package-lock-only`), never npm 11+.
+# See AGENTS.md §1 and scripts/verify-lockfile-npm10.mjs.
 ARG NODE_IMAGE=node:22-alpine@sha256:c610fcdfb1d5b4740dd70c284ed3cb16bb857e0f7166196e36a5501df7a3aa32
 FROM ${NODE_IMAGE} AS base
 WORKDIR /app
@@ -10,6 +13,10 @@ FROM base AS builder
 RUN apk --no-cache upgrade && apk --no-cache add python3 make g++ linux-headers
 
 COPY package.json package-lock.json ./
+# Fail fast with an actionable message if the lockfile was regenerated with
+# npm 11+ (drops the top-level @emnapi entries npm 10 requires). Without this,
+# `npm ci` still fails but with a cryptic "Missing: @emnapi/..." EUSAGE error.
+RUN node -e "const l=require('./package-lock.json');const p=l.packages||{};const miss=['node_modules/@emnapi/core','node_modules/@emnapi/runtime'].filter(k=>!p[k]);if(miss.length){console.error('LOCKFILE NOT npm-10-COMPATIBLE — missing: '+miss.join(', '));console.error('Regenerate with: npx -y npm@10.9.8 install --package-lock-only');console.error('See AGENTS.md §1.');process.exit(1)}"
 RUN --mount=type=cache,target=/root/.npm \
   npm ci