feat(orchestrator): reap stale claims via last_heartbeat_at

Dispatch loop now releases tasks whose holder has gone silent past
ROBOCO_CLAIM_HEARTBEAT_TTL_SECONDS (default 300s). Closes the
'dead container squats task forever' failure mode that the schema
hinted at but no code enforced.

roboco/config.py

@@ -305,6 +305,14 @@ class Settings(BaseSettings):
         ge=60,
         description="Claim heartbeat staleness threshold (seconds)",
     )
+    claim_heartbeat_ttl_seconds: int = Field(
+        default=300,
+        ge=60,
+        description=(
+            "If an agent's last_heartbeat_at is older than this, the dispatcher"
+            " considers the claim dead and releases the task back to pending."
+        ),
+    )
     spawn_cooldown_seconds: int = Field(
         default=60,
         ge=1,