hive-agent: add debug logging around todo upsert/clear/mark-done

diagnostic instrumentation for #2678 (phantom 'you have todos' wakes
after clearing bash-task todos). logs subsystem/key/id/changed on
UpsertTodo, subsystem/key/all/count on ClearTodo, id/count on
MarkTodoDone, and a marker when the serve loop actually consumes a
todo_wake notification. no behavior change - RUST_LOG=debug only.

hive-agent/src/main.rs

@@ -585,7 +585,10 @@ async fn serve_loop<S: Surface>(
                 }
             } {
                 RecvOutcome::Message(first) => first,
-                RecvOutcome::LocalTodo => synthetic_todo_message(),
+                RecvOutcome::LocalTodo => {
+                    tracing::debug!("todo wake consumed, sending synthetic todo message");
+                    synthetic_todo_message()
+                }
                 RecvOutcome::Empty => {
                     // Idle: no message this poll. Service a queued operator
                     // `/compact` here so it runs even when no turn is driving