Root cause: two separate silent-failure bugs caused 15s stalls on file-not-found:

1. agent-node/node.py _push_file(): when a READ arrives for a missing file,
   silently returned without sending any response. The Hub journal sat waiting
   for the full 15s timeout.
   Fix: send ERROR SyncStatus back immediately with task_id so the journal
   resolves at once.

2. ai-hub/assistant.py cat(): after the mirror fast-path found no file, it
   fell through and sent a READ gRPC to the node anyway, compounding the wait.
   Fix: short-circuit with {error: File not found} when the file is absent
   from the session mirror — no node round-trip needed.

test: added Case 9 (TestSmallFileSync) — asserts that cat() on a freshly
deleted file resolves in < 3s on both nodes (guard against regression).