feat: clean up session dir on session:stop event

[tupe12334]

Summary

• Architecture.md specified session cleanup on session:stop as planned but not yet implemented
• When polyhook emits session:stop, steplock now removes .steplock/sessions/<scope-key>/ and returns approve
• Falls back to reading .steplock/sessions/fallback-id when session_id is empty — no new fallback IDs created during cleanup
• If the scope dir doesn't exist (already cleaned or session never blocked), silently approves

Test plan

• cargo test passes (4 new tests covering all cleanup paths)
• session_stop_removes_scope_dir — creates a session, fires stop, verifies dir gone
• session_stop_approves_when_no_scope_dir — no error when dir absent
• session_stop_approves_when_no_steplock_dir — no error when .steplock/ absent
• session_stop_uses_fallback_id_when_session_id_empty — fallback-id path cleaned up

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