feat: worktree-aware gbrain code sources via path-hash IDs and CWD pin

Conductor sibling worktrees of the same repo no longer collide on a shared
gstack-code-<slug> source ID. /sync-gbrain now derives a path-hashed source
ID per worktree, runs gbrain sources attach to write .gbrain-source in the
worktree root, and removes the legacy unsuffixed source on first new-format
sync to prevent orphan accumulation.

Bug fixes surfaced by /codex during /ship:
- Silent attach failure now treated as stage failure (no more ok:true while
  pin is missing → unqualified code-def hits wrong source).
- Startup preamble checks .gbrain-source in the cwd worktree, not global
  state, so an unsynced worktree no longer claims "indexed" because a
  sibling synced.
- Code stage no longer skipped on remote-MCP (Path 4); the early-exit was
  in the SKILL template, not the orchestrator.
- Source registration routes through lib/gbrain-sources.ts only; deleted
  the near-duplicate ensureSourceRegisteredSync from the orchestrator.

Requires gbrain v0.30.0+ (uses sources attach). Phase 0 spike report:
~/.gstack/projects/garrytan-gstack/2026-05-08-gbrain-split-engine-spike.md

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitignore

@@ -18,6 +18,7 @@ bin/gstack-global-discover
 .openclaw/
 .hermes/
 .gbrain/
+.gbrain-source
 .context/
 extension/.auth.json
 # xterm assets are vendored from npm at build time; not source-of-truth.