Merge origin/main into garrytan/upgrade-gstack-gbrain-v1

Catch up to main (1.52.0.0, plan-tune cathedral + browse memory work).
Branch bumps to 1.52.1.0 — PATCH above main.

Conflict resolutions:
- VERSION / package.json → 1.52.1.0 (monotonic above main's 1.52.0.0)
- CHANGELOG.md → reconstructed reverse-chronological: this branch's
  brain-aware-planning + save-results entry renumbered 1.51.1.0 →
  1.52.1.0 on top, then main's 1.52.0.0 / 1.51.0.0 / 1.49.0.0 entries,
  then shared history. No entries dropped or orphaned.
- setup → kept both endgame blocks (my gbrain detection + main's
  plan-tune cathedral hook install); they're independent.
- SKILL.md files → regenerated from merged templates via
  bun run gen:skill-docs (canonical no-gbrain), not accepted from
  either merge side, per CLAUDE.md. Idempotent (0 STALE on re-run).
- bin/gstack-config → both sides' additions present (main's
  GSTACK_STATE_ROOT support + this branch's gbrain-refresh subcommand).

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "gstack",
-  "version": "1.51.1.0",
+  "version": "1.52.1.0",
   "description": "Garry's Stack — Claude Code skills + fast headless browser. One repo, one install, entire AI engineering workflow.",
   "license": "MIT",
   "type": "module",