test: 10 new cases close coverage gaps (helper defensive paths + migration)

/ship Step 7 coverage audit reported 48% (22/46 branches). Added 10 cases
covering the highest-impact gaps:

Helper (test/gstack-gbrain-source-wireup.test.ts, +3 cases → 19 total):
- --uninstall when gbrain is missing: best-effort exit 0, worktree still cleaned
- --no-pull skips HEAD advance on existing worktree (was untested)
- Stray non-git directory at worktree path is cleaned up + worktree created

Migration (test/gstack-upgrade-migration-v1_15_1_0.test.ts, NEW, 7 cases):
- HOME unset → defensive exit 0
- gbrain_sync_mode=off → exit 0 silently
- gbrain_sync_mode unset → exit 0 silently
- no ~/.gstack/.git → exit 0 silently
- helper missing on PATH → warning + exit 0
- happy path → invokes helper without --strict
- helper exits non-zero → migration prints retry hint, still exits 0 (non-blocking)

Also syncs package.json version from 1.15.0.0 → 1.15.1.0 to match VERSION
file (DRIFT_STALE_PKG repair from /ship Step 12 idempotency check; was a
manual-edit-bypass artifact from the merge step).

Coverage estimate: 48% → ~75%. Mainline + migration script + key defensive
paths all exercised. 26 tests total covering the new code surface.

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "gstack",
-  "version": "1.15.0.0",
+  "version": "1.15.1.0",
   "description": "Garry's Stack — Claude Code skills + fast headless browser. One repo, one install, entire AI engineering workflow.",
   "license": "MIT",
   "type": "module",