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>

sync-gbrain/SKILL.md.tmpl

@@ -226,7 +226,7 @@ gbrain status: GREEN
   Capability ...... OK   write+search round-trip
   CWD source ...... OK   <gstack-code-{repo_slug}> (page_count=<N>)
   ~/.gstack source. OK   <gstack-brain-{user}> (page_count=<N>) — managed by /setup-gbrain
-  Memory sync ..... OK   <gbrain_sync_mode>
+  Memory sync ..... OK   <artifacts_sync_mode>
   CLAUDE.md ....... OK   ## GBrain Search Guidance present
   Last sync ....... OK   <last_sync from state file>