fix(agents): reap stale active slots on cancel

[shiny-code-bot]

Summary

• Treat explicit agent cancel as a successful stale-slot cleanup when an active agent record remains but its task handle is already gone.
• Mark the stale record cancelled, stamp completion time, compact terminal state, and free active-agent capacity.
• Add a regression for the stale active-record/no-handle case.

Refs #214
Refs #124

Validation

• cargo test -p code-core cancel_agent_reaps_stale_active_record_without_handle
• ./build-fast.sh