fix: gstack-brain-sync advances the artifacts worktree after push

[alexputici]

Summary

gstack-brain-sync --once commits + pushes artifacts to the remote, but never advances the local detached worktree at ~/.gstack-brain-worktree. gbrain indexes from that worktree path, so the worktree stays at the init commit forever, gbrain sources list reports 0 pages for the artifacts source, and recent CEO plans / test plans / reviews never become searchable locally.

Reproduction

1. Enable artifacts sync (gstack-config set artifacts_sync_mode artifacts-only or full) and run gstack-artifacts-init.
2. Use gstack normally — run /autoplan, /ship, etc. so brain-sync runs --discover-new then --once, producing committed artifacts under ~/.gstack/projects/*/.
3. Inspect:

   git -C ~/.gstack worktree list
   # /Users/<you>/.gstack             <new SHA> [main]
   # /Users/<you>/.gstack-brain-worktree <init SHA> (detached HEAD)
   
   gbrain sources list | grep gstack-artifacts
   # ... 0 pages ...
   
   gbrain query "a phrase from your latest CEO plan"
   # No results.

The artifact files exist in ~/.gstack on main and on the GitHub remote. They just never reach the worktree gbrain scans.

Fix

Add a tiny advance_brain_worktree_to_head helper and call it on both successful-push paths in subcmd_once (the normal path and the fetch+merge retry path). It's a no-op when the worktree doesn't exist or the parent HEAD can't be read, so installs that haven't run gstack-gbrain-source-wireup yet are unaffected.

Why not call ensure_worktree from gstack-gbrain-source-wireup? That helper has additional side effects (source registration, jq probes, gbrain CLI calls) and lives in a script gstack-brain-sync doesn't currently source. A 12-line inline helper keeps the patch minimal and the push-success path side-effect-free on the gbrain CLI.

Test plan

• New test '--once advances the brain worktree HEAD to match parent after push' in test/brain-sync.test.ts: sets up a real detached worktree at the init commit (mirroring gstack-gbrain-source-wireup), runs --once against it, asserts the worktree HEAD matches the parent HEAD afterwards AND that the synced file appears on the worktree path.
• Verified the test fails without the fix (worktree stays at init SHA).
• bun test test/brain-sync.test.ts — 28/28 pass.
• bun test test/brain-sync.test.ts test/gbrain-sync-skip.test.ts test/gstack-gbrain-sync.test.ts — 53/53 pass.

Diff size

+17 in bin/gstack-brain-sync (helper + 2 call sites), +45 in test/brain-sync.test.ts (1 regression test). 2 files changed, 62 insertions.

🤖 Generated with Claude Code