fix(docs): rename stale gbrain_sync_mode to artifacts_sync_mode + register /document-generate

Five stale gstack-config references in docs/ pointed to the deprecated
gbrain_sync_mode key (renamed to artifacts_sync_mode in v1.27.0.0):
- docs/gbrain-sync.md: lines 62, 110, 111, 173
- docs/gbrain-sync-errors.md: lines 26, 203

Users following the docs would set a key that gstack-brain-sync no
longer reads, silently breaking artifacts sync.

Originated from @realcarsonterry PR #1461 (verbatim).

Also register /document-generate in AGENTS.md (Operational + memory
table) and docs/skills.md (skill index). The skill shipped in v1.35.0.0
but the doc-inventory cross-check in test/skill-validation.test.ts was
failing because neither file mentioned it.

Allowlist the new test/docs-config-keys.test.ts file in
test/no-stale-gstack-brain-refs.test.ts — it intentionally lists the
deprecated keys in its DEPRECATED_KEYS denylist (defending the rename).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

AGENTS.md

@@ -47,6 +47,7 @@ Invoke them by name (e.g., `/office-hours`).
 | `/canary` | Post-deploy monitoring loop using the browse daemon. |
 | `/landing-report` | Read-only dashboard for the workspace-aware ship queue. |
 | `/document-release` | Update all docs to match what you just shipped. |
+| `/document-generate` | Write missing docs from scratch in four Diataxis quadrants. |
 | `/setup-deploy` | One-time deploy config detection (Fly.io, Render, Vercel, etc.). |
 | `/gstack-upgrade` | Update gstack to the latest version. |