feat(gbrain-sync): queue primitives + writer shims

Adds bin/gstack-brain-enqueue (atomic append to sync queue) and
bin/gstack-jsonl-merge (git merge driver, ts-sort with SHA-256 fallback).
Wires one backgrounded enqueue call into learnings-log, timeline-log,
review-log, and developer-profile --migrate. question-log and
question-preferences stay local per Codex v2 decision.

gstack-config gains gbrain_sync_mode (off/artifacts-only/full) and
gbrain_sync_mode_prompted keys, plus GSTACK_HOME env alignment so
tests don't leak into real ~/.gstack/config.yaml.

bin/gstack-developer-profile

@@ -101,6 +101,10 @@ do_migrate() {
   mv "$TMPOUT" "$PROFILE_FILE"
   trap - EXIT
 
+  # gbrain-sync: enqueue the migrated file for cross-machine sync (no-op if off).
+  SCRIPT_DIR_E="$(cd "$(dirname "$0")" && pwd)"
+  "$SCRIPT_DIR_E/gstack-brain-enqueue" "developer-profile.json" 2>/dev/null &
+
   # Archive the legacy file.
   local TS
   TS="$(date +%Y-%m-%d-%H%M%S)"