Summary

• Clear the TUI input field before dispatching an inline /steer, so the submitted prompt can never linger in the buffer.
• Fixes the workflow hazard where a steered prompt could be accidentally re-submitted on the next Enter or turn-handoff.

Motivation

Closes #34569.

When /steer <prompt> is submitted while the agent is working, the Enter handler dispatched the command and reset the input buffer afterward. If process_command() re-entered the prompt_toolkit event loop (a print routed through run_in_terminal, or a modal that snapshots+restores the in-progress draft) or raised before returning, the post-dispatch reset() was skipped — leaving the submitted prompt sitting in the input field. The next Enter (or the auto-resume / next-turn handoff) could then re-send it, which is the opposite of /steer's mid-run-injection purpose.

The fix routes the inline-steer case through a small _dispatch_inline_steer() helper that resets the buffer before running process_command(). This makes the TUI's input-state contract — "what's in the box hasn't been sent" — hold unconditionally, regardless of how process_command behaves. As a bonus, because the buffer is now empty before any modal opens, the modal draft snapshot/restore path (_capture_modal_input_snapshot / _restore_modal_input_snapshot) can no longer resurrect the submitted steer text.

This matches the existing post-submit clear contract that normal prompts already get, and addresses the issue's hypothesis #3 (race between async steer consumption and the synchronous buffer clear).

Verification

• python3 -m pytest tests/cli/test_cli_steer_busy_path.py — 9 passed (7 existing + 2 new)
• New tests:
  • test_buffer_cleared_before_dispatch — asserts the buffer is empty at the moment process_command runs, the submitted text was appended to history, the steer still reaches agent.steer(), and the final buffer is empty.
  • test_buffer_cleared_even_if_dispatch_raises — asserts the buffer is still cleared when process_command raises (the exact case a post-dispatch reset would miss).
• Did NOT change: steer semantics, the idle-path / queue / interrupt fallbacks, or the detector gate — only the ordering of the buffer reset relative to dispatch on the inline path.

Notes

The change is a single small refactor: the inline-steer block in the Enter handler now calls self._dispatch_inline_steer(buffer, text), which does buffer.reset(append_to_history=True) then process_command(text).