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_tier4_e2e.py

@@ -432,7 +432,7 @@ d['herdr_sessions'] = [
         # Run run_loop.sh
         cmd_loop = [
             "bash", str(loop_script),
-            "--target-agent", worker_name,
+            "--creator", worker_name,
             "--reviewer", reviewer_name,
             "--plan",
             "--plan-talk", "1",