feat: add tool_result_persist hook

docs/concepts/agent-loop.md

@@ -73,6 +73,7 @@ These run inside the agent loop or gateway pipeline:
 - **`agent_end`**: inspect the final message list and run metadata after completion.
 - **`before_compaction` / `after_compaction`**: observe or annotate compaction cycles.
 - **`before_tool_call` / `after_tool_call`**: intercept tool params/results.
+- **`tool_result_persist`**: synchronously transform tool results before they are written to the session transcript.
 - **`message_received` / `message_sending` / `message_sent`**: inbound + outbound message hooks.
 - **`session_start` / `session_end`**: session lifecycle boundaries.
 - **`gateway_start` / `gateway_stop`**: gateway lifecycle events.