feat(skills): add cleanup-artifacts skill and session hygiene protocol

protocols/workflow.md

@@ -43,3 +43,10 @@ Choose checks that can detect a real failure in the changed behavior.
 
 Complete means the requested behavior is implemented and supported by empirical
 evidence. A successful command exit alone does not prove the user-facing outcome.
+
+## Session Hygiene
+
+After task verification and handoff are complete, offer or execute (upon user
+confirmation) session cleanup via `cleanup-artifacts`. Purge scratch scripts,
+stale test caches, and harness session bloat across Antigravity, Claude Code,
+Codex, and OpenCode while strictly preserving rules, custom skills, and MCP tools.