chore: gitignore .factory/ directory

Generated by `bun run gen:skill-docs --host factory` (Factory Droid
support from v0.13.5.0). Same pattern as .agents/ for Codex.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

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