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-errors.md

@@ -23,7 +23,7 @@ This pulls the repo into `~/.gstack/` and re-registers merge drivers.
 
 If you don't want to restore here, dismiss the hint with:
 ```bash
-gstack-config set gbrain_sync_mode_prompted true
+gstack-config set artifacts_sync_mode_prompted true
 ```
 
 ---
@@ -200,7 +200,7 @@ canonical config files from the brain repo.
 
 1. `gstack-brain-sync --status` — is mode `off`?
 2. `~/.gstack/.git` exists?
-3. `gstack-config get gbrain_sync_mode` — should be `full` or `artifacts-only`.
+3. `gstack-config get artifacts_sync_mode` — should be `full` or `artifacts-only`.
 4. The file you expect to sync — is it in the allowlist?
    `cat ~/.gstack/.brain-allowlist`
 5. Privacy class filter — if mode is `artifacts-only`, behavioral files