chore: gitignore .factory/ and remove from tracking

Factory Droid support was removed in this branch. The .factory/ directory
was re-added by merging main (which had v0.13.5.0 Factory support).
Gitignore it so it stays out.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -6,6 +6,7 @@ bin/gstack-global-discover
 .gstack/
 .claude/skills/
 .agents/
+.factory/
 .context/
 extension/.auth.json
 .gstack-worktrees/