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>

docs/gbrain-sync.md

@@ -59,7 +59,7 @@ privacy mode:
 - **Only artifacts**: plans, designs, retros, learnings — skip
   behavioral data (timelines, developer profile).
 - **Decline**: keep everything local. You can turn sync on later with
-  `gstack-config set gbrain_sync_mode full`.
+  `gstack-config set artifacts_sync_mode full`.
 
 Your answer is persisted. You won't be asked again.
 
@@ -107,8 +107,8 @@ output. Scan it for problems.
 
 Change anytime with:
 ```bash
-gstack-config set gbrain_sync_mode full
-gstack-config set gbrain_sync_mode off
+gstack-config set artifacts_sync_mode full
+gstack-config set artifacts_sync_mode off
 ```
 
 ## Secret protection
@@ -170,7 +170,7 @@ gstack-brain-uninstall
 This:
 
 - Removes `~/.gstack/.git/` and all `.brain-*` config files.
-- Clears `gbrain_sync_mode` in `gstack-config`.
+- Clears `artifacts_sync_mode` in `gstack-config`.
 - Does NOT touch your learnings, plans, retros, or developer profile.
 
 Add `--delete-remote` to also delete the private GitHub repo (GitHub only,