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>

bin/gstack-config

@@ -8,11 +8,13 @@
 #   gstack-config defaults           — show just the defaults table
 #
 # Env overrides (for testing):
+#   GSTACK_STATE_ROOT — override ~/.gstack state directory (highest priority,
+#                       matches D16 cathedral isolation convention)
 #   GSTACK_HOME       — override ~/.gstack state directory (aligns with writer scripts)
 #   GSTACK_STATE_DIR  — legacy alias for GSTACK_HOME (kept for backwards compat)
 set -euo pipefail
 
-STATE_DIR="${GSTACK_HOME:-${GSTACK_STATE_DIR:-$HOME/.gstack}}"
+STATE_DIR="${GSTACK_STATE_ROOT:-${GSTACK_HOME:-${GSTACK_STATE_DIR:-$HOME/.gstack}}}"
 CONFIG_FILE="$STATE_DIR/config.yaml"
 
 # Annotated header for new config files. Written once on first `set`.