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>

bin/gstack-config

@@ -60,8 +60,8 @@ CONFIG_HEADER='# gstack configuration — edit freely, changes take effect on ne
 #                           # Unknown values default to "default" with a warning.
 #                           # See docs/designs/PLAN_TUNING_V1.md for rationale.
 #
-# ─── GBrain sync (v1.7+) ─────────────────────────────────────────────
-# gbrain_sync_mode: off     # off | artifacts-only | full
+# ─── Artifacts sync (renamed from gbrain_sync_mode in v1.27.0.0) ─────
+# artifacts_sync_mode: off  # off | artifacts-only | full
 #                           #   off            — no sync (default)
 #                           #   artifacts-only — sync plans/designs/retros/learnings only
 #                           #                    (skip behavioral data: question-log,
@@ -69,7 +69,7 @@ CONFIG_HEADER='# gstack configuration — edit freely, changes take effect on ne
 #                           #   full           — sync everything allowlisted
 #                           # Set by the first-run privacy stop-gate. See docs/gbrain-sync.md.
 #
-# gbrain_sync_mode_prompted: false
+# artifacts_sync_mode_prompted: false
 #                           # Set to true once the privacy gate has asked the user.
 #                           # Flip back to false to be re-prompted.
 #
@@ -105,8 +105,8 @@ lookup_default() {
     skip_eng_review) echo "false" ;;
     workspace_root) echo "$HOME/conductor/workspaces" ;;
     cross_project_learnings) echo "" ;; # intentionally empty → unset triggers first-time prompt
-    gbrain_sync_mode) echo "off" ;;
-    gbrain_sync_mode_prompted) echo "false" ;;
+    artifacts_sync_mode) echo "off" ;;
+    artifacts_sync_mode_prompted) echo "false" ;;
     *) echo "" ;;
   esac
 }
@@ -138,8 +138,8 @@ case "${1:-}" in
       echo "Warning: explain_level '$VALUE' not recognized. Valid values: default, terse. Using default." >&2
       VALUE="default"
     fi
-    if [ "$KEY" = "gbrain_sync_mode" ] && [ "$VALUE" != "off" ] && [ "$VALUE" != "artifacts-only" ] && [ "$VALUE" != "full" ]; then
-      echo "Warning: gbrain_sync_mode '$VALUE' not recognized. Valid values: off, artifacts-only, full. Using off." >&2
+    if [ "$KEY" = "artifacts_sync_mode" ] && [ "$VALUE" != "off" ] && [ "$VALUE" != "artifacts-only" ] && [ "$VALUE" != "full" ]; then
+      echo "Warning: artifacts_sync_mode '$VALUE' not recognized. Valid values: off, artifacts-only, full. Using off." >&2
       VALUE="off"
     fi
     mkdir -p "$STATE_DIR"
@@ -171,7 +171,7 @@ case "${1:-}" in
     for KEY in proactive routing_declined telemetry auto_upgrade update_check \
                skill_prefix checkpoint_mode checkpoint_push codex_reviews \
                gstack_contributor skip_eng_review workspace_root \
-               gbrain_sync_mode gbrain_sync_mode_prompted; do
+               artifacts_sync_mode artifacts_sync_mode_prompted; do
       VALUE=$(grep -E "^${KEY}:" "$CONFIG_FILE" 2>/dev/null | tail -1 | awk '{print $2}' | tr -d '[:space:]' || true)
       SOURCE="default"
       if [ -n "$VALUE" ]; then
@@ -187,7 +187,7 @@ case "${1:-}" in
     for KEY in proactive routing_declined telemetry auto_upgrade update_check \
                skill_prefix checkpoint_mode checkpoint_push codex_reviews \
                gstack_contributor skip_eng_review workspace_root \
-               gbrain_sync_mode gbrain_sync_mode_prompted; do
+               artifacts_sync_mode artifacts_sync_mode_prompted; do
       printf '  %-24s %s\n' "$KEY:" "$(lookup_default "$KEY")"
     done
     ;;