Force-end every active sim session for an agent (runaway recovery).

DB-authoritative, Durable-Object-independent recovery for a stuck/runaway Dojo session: aborts every non-terminal (pending/running/paused) dojo_sim_sessions row for the agent directly in the control plane, clearing the one-active-session-per-agent lock even when the per-session runner DO has crashed or been evicted (the ghost-DO case /sim/{session_id}/stop cannot resolve, because that path no-ops the DB when the DO has no in-memory state). Keyed by agent_id because a locked-out caller holds the agent id, not the orphaned session id. Idempotent: returns { ok: true, ended: 0 } when nothing was active. MNE-531.