feat(runtime): heartbeat immediately after claim

Assistant: codex
Assistant-Model: gpt-5.6-sol
Assistant-Session: 01a06ba0-10aa-7ea0-b20a-4f3fac39efe9

rein_aharness/claim_loop.py

@@ -305,6 +305,30 @@ def process_one(
         return ProcessResult(claimed=False, empty=True, reason="queue empty")
 
     run = claimed[0]
+    try:
+        # Make active lease ownership observable even for normal short jobs,
+        # and fail closed before dispatch if Activity Core no longer accepts it.
+        client.heartbeat(run.id, lease_seconds=cfg.lease_seconds)
+        logger.info("accepted initial heartbeat run_id=%s", run.id)
+    except OpsRunError as exc:
+        rejected = exc.status_code == 409
+        detail_key = "lease_loss" if rejected else "heartbeat_failure"
+        return ProcessResult(
+            claimed=True,
+            run_id=run.id,
+            ok=False,
+            retry_full_interval=True,
+            reason=(
+                f"initial heartbeat {'rejected' if rejected else 'failed'}: {exc}"
+            ),
+            detail={
+                detail_key: {
+                    "stage": "pre-dispatch-heartbeat",
+                    "status_code": exc.status_code,
+                    "code": exc.code,
+                }
+            },
+        )
     approach = GLAS_APPROACH if run.harness_profile_ref else select_approach(run)
     logger.info(
         "claimed run_id=%s approach=%s title=%r labels=%s",