refactor: rename gbrain_sync_mode → artifacts_sync_mode (v1.27.0.0 prep)

Hard rename, no dual-read alias (codex Finding D4). The on-disk migration
script (Phase C, separate commit) renames the config key in users'
~/.gstack/config.yaml and any CLAUDE.md blocks.

Touched call sites:
- bin/gstack-config defaults + validation + list/defaults output
- bin/gstack-gbrain-detect (gstack_brain_sync_mode field still emitted
  with the same name for downstream-tool compat; reads new key)
- bin/gstack-brain-sync, bin/gstack-brain-enqueue, bin/gstack-brain-uninstall
- bin/gstack-timeline-log (comment ref)
- scripts/resolvers/preamble/generate-brain-sync-block.ts: renames key,
  branches on gbrain_mcp_mode=remote-http to emit "ARTIFACTS_SYNC:
  remote-mode (managed by brain server <host>)" instead of the local
  mode/queue/last_push line (codex Finding #11)
- bin/gstack-brain-restore + bin/gstack-gbrain-source-wireup: read
  ~/.gstack-artifacts-remote.txt with ~/.gstack-brain-remote.txt fallback
  during the migration window
- bin/gstack-artifacts-init: tolerant of unrecognized URL forms (local
  paths, file://, self-hosted gitea) so test infrastructure and unusual
  remotes work without canonicalization
- test/brain-sync.test.ts: gstack-brain-init → gstack-artifacts-init
- test/skill-e2e-brain-privacy-gate.test.ts: artifacts_sync_mode keys
- test/gen-skill-docs.test.ts: budget 35K → 36.5K for the new MCP-mode
  probe in the preamble resolver
- health/SKILL.md.tmpl, sync-gbrain/SKILL.md.tmpl: comment + verdict line

Hard delete:
- bin/gstack-brain-init (replaced by bin/gstack-artifacts-init in v1.27.0.0)
- test/gstack-brain-init-gh-mock.test.ts (replaced by gstack-artifacts-init.test.ts)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/skill-e2e-brain-privacy-gate.test.ts

@@ -4,7 +4,7 @@
  * The gbrain-sync preamble block instructs the model to fire a one-time
  * AskUserQuestion when:
  *   - `BRAIN_SYNC: off` in the preamble echo (sync mode not on)
- *   - config `gbrain_sync_mode_prompted` is "false"
+ *   - config `artifacts_sync_mode_prompted` is "false"
  *   - gbrain is detected on the host (binary on PATH or `gbrain doctor`
  *     --fast --json succeeds)
  *
@@ -31,14 +31,14 @@ const describeE2E = shouldRun ? describe : describe.skip;
 
 describeE2E('gbrain-sync privacy gate fires once via preamble', () => {
   test('gstack skill preamble fires the 3-option AskUserQuestion when gbrain is detected', async () => {
-    // Stage a fresh GSTACK_HOME with gbrain_sync_mode_prompted=false.
+    // Stage a fresh GSTACK_HOME with artifacts_sync_mode_prompted=false.
     const gstackHome = fs.mkdtempSync(path.join(os.tmpdir(), 'privacy-gate-gstack-'));
     const fakeBinDir = fs.mkdtempSync(path.join(os.tmpdir(), 'privacy-gate-bin-'));
 
     // Seed the config so the gate's condition passes.
     fs.writeFileSync(
       path.join(gstackHome, 'config.yaml'),
-      'gbrain_sync_mode: off\ngbrain_sync_mode_prompted: false\n',
+      'artifacts_sync_mode: off\nartifacts_sync_mode_prompted: false\n',
       { mode: 0o600 }
     );
 
@@ -151,14 +151,14 @@ describeE2E('gbrain-sync privacy gate fires once via preamble', () => {
     }
   }, 180_000);
 
-  test('privacy gate does NOT fire when gbrain_sync_mode_prompted is already true', async () => {
+  test('privacy gate does NOT fire when artifacts_sync_mode_prompted is already true', async () => {
     // Same staging, but prompted=true this time. Gate should be silent.
     const gstackHome = fs.mkdtempSync(path.join(os.tmpdir(), 'privacy-gate-off-'));
     const fakeBinDir = fs.mkdtempSync(path.join(os.tmpdir(), 'privacy-gate-off-bin-'));
 
     fs.writeFileSync(
       path.join(gstackHome, 'config.yaml'),
-      'gbrain_sync_mode: off\ngbrain_sync_mode_prompted: true\n',
+      'artifacts_sync_mode: off\nartifacts_sync_mode_prompted: true\n',
       { mode: 0o600 }
     );