chore: ignore gbrain .sources/ local staging dir

gbrain writes per-source staging and capability-check artifacts under
.sources/ in the repo root. It's machine-local runtime state, not source.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -37,3 +37,6 @@ supabase/.temp/
 
 # Throughput analysis — local-only, regenerate via scripts/garry-output-comparison.ts
 docs/throughput-*.json
+
+# gbrain local source-staging dir (capability checks, source clones) — runtime artifact
+.sources/