Add plan-mode pending guard to enforce enter_plan_mode after reading plan_sop

[HamsteRider-m]

What

When plan_sop.md is read, a pending plan-mode flag is now set. If the agent does not call handler.enter_plan_mode(...) in the same turn, the turn-end callback injects a hard guard that blocks normal execution until plan mode is properly entered. Once enter_plan_mode() succeeds, the pending state is cleared and all existing in-plan-mode checks take over.

This prevents the agent from reading the SOP, extracting its points, and then continuing ordinary execution without ever creating a plan file or entering plan mode.

Changes

• ga.py: set plan_mode_pending on plan SOP read; inject guard in turn_end_callback() when pending but not yet in plan mode; clear pending on enter_plan_mode().
• test_plan_mode_guard.py: three test cases covering pending flag set, guard interception, and pending cleared on entry.

Verification

python -m pytest -q test_plan_mode_guard.py   # 3 passed
python -m py_compile ga.py test_plan_mode_guard.py   # ok

Closes #458