- Add in_plan_mode flag to Agent struct
- Add set_plan_mode() and is_plan_mode() methods
- Gate check now only runs when in_plan_mode is true
- CLI calls set_plan_mode(true) on /plan command and EnterPlanMode
- CLI calls set_plan_mode(false) on approval and CTRL-D exit
- Update integration test to enable plan mode
- Fix test YAML to use Vec<Check> for negative/boundary checks