feat: Complete Pipeline Mutation API implementation

- Add can_hot_swap() function to Pipeline class
- Add cleanup_stage() method to Pipeline class
- Fix remove_stage() to rebuild execution order after removal
- Extend ui_panel.execute_command() with docstrings for mutation commands
- Update WebSocket handler to support pipeline mutation commands
- Add _handle_pipeline_mutation() function for command routing
- Add comprehensive integration tests in test_pipeline_mutation_commands.py
- Update AGENTS.md with mutation API documentation

Issue: #35 (Pipeline Mutation API)
Acceptance criteria met:
- ✅ can_hot_swap() checker for stage compatibility
- ✅ cleanup_stage() cleans up specific stages
- ✅ remove_stage_safe() rebuilds execution order (via remove_stage)
- ✅ Unit tests for all operations
- ✅ Integration with WebSocket commands
- ✅ Documentation in AGENTS.md

engine/pipeline/ui.py

@@ -370,13 +370,24 @@ class UIPanel:
     def execute_command(self, command: dict) -> bool:
         """Execute a command from external control (e.g., WebSocket).
 
-        Supported commands:
+        Supported UI commands:
         - {"action": "toggle_stage", "stage": "stage_name"}
         - {"action": "select_stage", "stage": "stage_name"}
         - {"action": "adjust_param", "stage": "stage_name", "param": "param_name", "delta": 0.1}
         - {"action": "change_preset", "preset": "preset_name"}
         - {"action": "cycle_preset", "direction": 1}
 
+        Pipeline Mutation commands are handled by the WebSocket/runner handler:
+        - {"action": "add_stage", "stage": "stage_name", "type": "source|display|camera|effect"}
+        - {"action": "remove_stage", "stage": "stage_name"}
+        - {"action": "replace_stage", "stage": "old_stage_name", "with": "new_stage_type"}
+        - {"action": "swap_stages", "stage1": "name1", "stage2": "name2"}
+        - {"action": "move_stage", "stage": "stage_name", "after": "other_stage"|"before": "other_stage"}
+        - {"action": "enable_stage", "stage": "stage_name"}
+        - {"action": "disable_stage", "stage": "stage_name"}
+        - {"action": "cleanup_stage", "stage": "stage_name"}
+        - {"action": "can_hot_swap", "stage": "stage_name"}
+
         Returns:
             True if command was handled, False if not
         """