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

tests/test_o2_race_free_lock.py

@@ -187,7 +187,7 @@ def test_o2_12_run_loop_exits_on_lock_failure(tmp_path):
     marker = tmp_path / "loop-guard-active"
     proc = acquire_bg(marker)
     try:
-        cmd = ["bash", str(RUN_LOOP), "--target-agent", "dummy-agent", "--task", "test"]
+        cmd = ["bash", str(RUN_LOOP), "--creator", "dummy-agent", "--task", "test"]
         run_env = dict(os.environ)
         run_env["MAM_LOOP_MARKER"] = str(marker)
         res = subprocess.run(cmd, capture_output=True, text=True, cwd=str(tmp_path), env=run_env)