feat(agent): emit structured _tool_events progress metadata

Extend the existing on_progress callback to carry structured tool-event
payloads alongside the plain-text hint, so channels can render rich
tool execution state (start/finish/error, arguments, results, file
attachments) rather than only the pre-formatted hint string.

Changes
-------
- AgentLoop._tool_event_start_payload() — builds a version-1 start
  payload from a ToolCallRequest
- AgentLoop._tool_event_result_extras() — extracts files/embeds from a
  tool result dict
- AgentLoop._tool_event_finish_payloads() — maps tool_calls +
  tool_results + tool_events from AgentHookContext into finish payloads
- _LoopHook.before_execute_tools() — passes tool_events=[...] to
  on_progress together with the existing tool_hint flag
- _LoopHook.after_iteration() — emits a second on_progress call with
  the finish payloads once tool results are available
- _bus_progress() — forwards tool_events as _tool_events in OutboundMessage
  metadata so channel implementations can read them
- on_progress type widened to Callable[..., Awaitable[None]] on all
  public entry points; _cli_progress updated to accept and ignore
  tool_events

The contract is additive: callers that only accept (content, *, tool_hint)
continue to work unchanged. Callers that also accept tool_events receive
the structured data.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nanobot/cli/commands.py

@@ -1028,7 +1028,7 @@ def agent(
     # Shared reference for progress callbacks
     _thinking: ThinkingSpinner | None = None
 
-    async def _cli_progress(content: str, *, tool_hint: bool = False) -> None:
+    async def _cli_progress(content: str, *, tool_hint: bool = False, **_kwargs: Any) -> None:
         ch = agent_loop.channels_config
         if ch and tool_hint and not ch.send_tool_hints:
             return