Add stdin stream mode for the cli

[cte]

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[roomote]

Rooviewer   See task

All issues resolved. Both the prompt echo role inconsistency and the state leak fix look correct.

• Prompt echo partial messages have inconsistent role in stream-json mode (json-event-emitter.ts)
• JsonEventEmitter state leaks across tasks in --stdin-prompt-stream mode (run.ts)

Previous reviews

• 46cd9c4: Review #1
• 4d9a466: Review #2

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}