fix(agent): wait for prompt enter submission

[charlieYong]

Summary

• agent.prompt could return / --wait could resolve while prompt text still sat unsubmitted in the composer, because delayed Enter was scheduled separately and composer text alone could advance state_change_seq.
• Prompt text and delayed Enter are now one ordered PTY submission: intervening input is deferred until Enter is written.
• --wait sleeps through the Enter delay, then rebaselines effect observation on a post-Enter state_change_seq so pre-Enter screen updates cannot satisfy the effect gate. Callers without --wait still return immediately after enqueue.

refs #2422

Test plan

• ordered_submission_* pane tests (interleave deferred; second hold rejected)
• agent_prompt_sends_text_then_delays_enter and agent_prompt_orders_enter_before_intervening_input
• prompt_wait_rebaseline_rejects_composer_only_seq_bump
• cargo fmt --check + cargo clippy -D warnings
• all herdr binary unit tests (3035)
• full just ci in this environment: PTY-spawned integration tests (auto_detect_*, some session/client_mode/multi_client) also fail on clean master here; not caused by this patch

Made with Cursor

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Hi @charlieYong, thanks for your interest in contributing.

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