feat(mux-loop): redesign role CLI flags with --creator and --planner, drop --target-agent

- Replace '--target-agent' with mandatory '--creator <session>' flag
- Add explicit rejection error for legacy '--target-agent'
- Add '--planner <session>' flag with fail-fast check when '--plan' is missing
- Implement 2-branch session validation ('is not registered' vs 'is not running')
- Update in-repo references in hooks, SKILL.md, INSTALL.md, and tests
- Add tests/test_loop_cli.py with 9 unit/contract tests

.agents/hooks/loop_delegation_guard.sh

@@ -131,6 +131,6 @@ emit("deny",
      "The /multi-agent-mux-loop skill is active, so direct file edits are out "
      "of scope for the orchestrator. Stop editing and delegate instead: run "
      "bash .agents/skills/multi-agent-mux-loop/scripts/run_loop.sh "
-     "--target-agent <session> --task <goal>. "
+     "--creator <session> --task <goal>. "
      "See .agents/MULTI_AGENT_RULES.md #3.2 (Invocation-Aware Scoped Guard).")
 PY