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_o3_scoped_guard.py

@@ -379,7 +379,7 @@ def test_b13_reexec_preserves_original_argv(tmp_path):
     """
     run_loop_sh = REPO_ROOT / ".agents" / "skills" / "multi-agent-mux-loop" / "scripts" / "run_loop.sh"
     res = subprocess.run(
-        ["bash", str(run_loop_sh), "--target-agent", "nonexistent-agent", "--task", "test goal with spaces"],
+        ["bash", str(run_loop_sh), "--creator", "nonexistent-agent", "--task", "test goal with spaces"],
         capture_output=True,
         text=True,
         cwd=str(REPO_ROOT),
@@ -461,7 +461,7 @@ def test_b13_no_freeze_switch_disables_reexec(tmp_path):
     env = os.environ.copy()
     env["MAM_LOOP_NO_FREEZE"] = "1"
     res = subprocess.run(
-        ["bash", str(run_loop_sh), "--target-agent", "nonexistent-agent", "--task", "test goal"],
+        ["bash", str(run_loop_sh), "--creator", "nonexistent-agent", "--task", "test goal"],
         capture_output=True,
         text=True,
         env=env,