fix: correct task scheduling and simplify agent internals

Fix queue cancellation, concurrency limits and worker status updates.

Use direct Explorer imports, split the Diagnoser loop into helpers and remove unused tool registration and prompt-builder code.

Correct model caching, endpoint selection and retry handling.

artemis/runtime/trace_store.py

@@ -326,6 +326,23 @@ def update_trace_status(
         return data
 
 
+def update_trace_pid(trace_id: str, pid: int) -> dict[str, Any] | None:
+    """Updates the pid field of the status.json for a given trace_id."""
+    path = get_status_path(trace_id)
+    with _status_lock(path):
+        data = read_status(trace_id)
+        if not data:
+            if os.path.exists(path) or os.path.exists(f"{path}.corrupt"):
+                logger.warning(
+                    f"Dropping pid update for trace {trace_id}: existing status.json is unreadable"
+                )
+            return None
+
+        data["pid"] = pid
+        write_status(trace_id, data)
+        return data
+
+
 def update_trace_device_serial(trace_id: str, device_serial: str) -> dict[str, Any] | None:
     """Updates the device_serial field of the status.json for a given trace_id."""
     path = get_status_path(trace_id)