- PLAN_EXEC branch builds PhasePolicy from ServerConfig.plan_exec
- Empty config → default_policy(); enabled=False → falls back to REACT
- Bad config → error event sent, returns early (no engine constructed)
- ReActEngine created with phase_policy; AdvancePhaseTool registered
- phase_changed events emitted on phase transitions (PLAN_EXEC only)
- REST send_message with execution_mode=plan_exec → HTTP 501 (KTD4)
- REWOO/REFLEXION/TEAM_COLLAB still fall back to REACT (no regression)
- 9 unit tests covering REST 501, characterization, happy path, edge cases, error path, phase_changed events
chiguyong