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/gen-skill-docs.test.ts

@@ -313,15 +313,17 @@ describe('gen-skill-docs', () => {
     ];
 
     // Plan skills carry the same preamble surface as other tier-≥2 skills
-    // (Brain Sync, Context Recovery, Routing Injection are load-bearing
+    // (Artifacts Sync, Context Recovery, Routing Injection are load-bearing
     // functionality, not optional). Budget is set to current size + small
     // headroom; ratchet down if a future slim trims real bytes.
     // Ratcheted from 33000 → 35000 when the gbrain context-load block was
-    // added to generate-brain-sync-block.ts (per /sync-gbrain plan §4).
+    // added (per /sync-gbrain plan §4). Ratcheted 35000 → 36500 in v1.27.0.0
+    // when generate-brain-sync-block.ts gained the gbrain_mcp_mode probe +
+    // remote-mode ARTIFACTS_SYNC status line (Path 4 of /setup-gbrain).
     for (const skill of reviewSkills) {
       const content = fs.readFileSync(skill.path, 'utf-8');
       const preamble = extractPreambleBeforeWorkflow(content, skill.markers);
-      expect(Buffer.byteLength(preamble, 'utf-8')).toBeLessThan(35_000);
+      expect(Buffer.byteLength(preamble, 'utf-8')).toBeLessThan(36_500);
     }
   });