chore: gitignore .factory/ — generated output, not source

Same pattern as .claude/skills/ and .agents/. These SKILL.md files are
generated from .tmpl templates by gen:skill-docs --host factory.

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/