[FEATURE] Stream subagent output to parent terminal session in real time

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When a background subagent is running, its output is only available on completion or by tailing a JSONL file in a separate terminal. For long-running agents (e.g. polling a CI/CD
pipeline), there's no way to observe progress in the main session. Request: stream subagent output into the parent terminal in real time — even a simple prefixed log line per tool call
would be sufficient.

Proposed Solution

A stream_to_parent option on background agents. When enabled, each tool call and result is forwarded as a compact status line to the parent terminal in real time:

[agent] Bash: checking pipeline status...
[agent] → stage: running
[agent] Bash: sleeping 30s...
[agent] → stage: success, triggering prod...

The parent conversation wouldn't be interrupted — these would be informational lines only. The final completion notification would still work the same way.

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

CLI commands and flags

Use Case Example

Any long-running polling task (CI/CD pipelines, waiting for external state changes) where the user wants live progress without switching terminals or waiting for completion.

Additional Context

_No response_