Replaces hardcoded 0 values in the Impact panel of `agents actor remove`
with real DB-backed counts:
- Sessions: counts sessions whose `actor_name` matches the removed actor
- Active Plans: counts lifecycle plans in queued/processing state that
reference the actor via `strategy_actor` or `execution_actor`
- Actions Referencing: counts actions that reference the actor in any
actor field (strategy, execution, review, apply, estimation, invariant)
The new `_compute_actor_impact()` helper queries each subsystem via the
DI container and returns (session_count, active_plan_count, action_count).
All queries are wrapped in broad exception handlers so that DB unavailability
never blocks the removal itself.
Adds a BDD feature file and step definitions that verify the Impact panel
shows real non-zero counts when the actor is referenced, and zero counts
when it is not.
Removes the misleading comment about 'conservative estimates'.
ISSUES CLOSED: #3420
freemo