feat: Stream subagent/dynamic_task output to CLI terminal

[clouatre]

Please explain the motivation behind the feature request.

When using dynamic_task__create_task or sub_recipes to spawn subagents, their execution is opaque. The CLI only shows:

─── execute_task | subagent ──────────────────────────
task_ids: 20251218_46

The subagent runs successfully (verified in sessions.db), but users cannot see what it is doing. This makes multi-agent workflows difficult to monitor and debug.

Describe the solution you'd like

Add a stream_output parameter to dynamic_task__create_task:

{
  "task_parameters": [{
    "title": "BUILD Phase",
    "instructions": "...",
    "stream_output": true,
    "return_last_only": true
  }]
}

When stream_output: true, subagent tool calls and responses stream to the parent terminal in real-time. This is independent of return_last_only, which controls what returns to the orchestrator context.

Describe alternatives you've considered

• Global CLI flag (--verbose-subagents)
• Environment variable (GOOSE_STREAM_SUBAGENTS=true)
• Query sessions.db after execution
• Handoff files for inter-agent communication

Additional context

• Documentation mentions "Real-Time Progress Monitoring" for parallel subrecipes, but this does not apply to sequential tasks

• Related: Improve observability (feedback from a new user) #6052 (general observability)

• I have verified this does not duplicate an existing feature request

[clouatre]

After investigating the codebase:

Current state:

• subagent_handler.rs iterates AgentEvent::Message from the subagent reply stream (lines 197-209) but discards them
• subagent_tool.rs sets notification_stream: None (line 223)
• The streaming pattern exists in extension_manager.rs: create mpsc::channel, wrap in ReceiverStream, set as notification_stream

Related:

• PR refactor: subagents as platform extension (enables subagents in code mode) #6160 refactors subagents to a platform extension and renames ToolCallResult to DeferredToolCall - the notification_stream field remains but is still set to None
• This feature should be a follow-up PR after refactor: subagents as platform extension (enables subagents in code mode) #6160 merges

Implementation path:

1. Add mpsc::Sender<ServerNotification> parameter to run_complete_subagent_task
2. Forward AgentEvent::Message to the sender in the event loop
3. Create channel in handle_subagent_tool, set notification_stream to receiver
4. CLI already handles notifications via task_execution_display

@tlongwell-block - I'd be happy to submit a PR for this once #6160 is merged. Does this approach align with your plans?

[rabi]

@clouatre #6535 would probably address this?