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: #6052 (general observability)

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