mecatui: queued steer card implies Esc cancels the steer, but it cancels the run and falsely shows the steer as applied

Bug description

When a steer is queued during an active mecatui run, its card says esc cancel. Pressing Escape does not cancel/retract the steer: it cancels the active run.

The resulting state is also misleading. Run termination records the pending steer into session history and the TUI renders the steer echo as steer applied, even though the cancelled run performs no subsequent model turn that could process it. The steer is only available in history for a later run.

Steps to reproduce

1. Start a prompt in mecatui that keeps the run active long enough to steer.
2. Submit another prompt while the run is active.
3. Wait for the steer queued · ↑ edit · esc cancel card.
4. Press Escape.
5. Observe that the active run is cancelled and the steer appears to have been applied, although the model did not process it.

Expected behavior

The contextual action and resulting status should distinguish clearly between retracting the queued steer and cancelling the active run.

If Escape is intended to cancel the run, the card should say so explicitly, and a steer captured at cancellation should be presented as saved for a future run rather than applied. Alternatively, if esc cancel refers to the card's queued steer, Escape should retract that steer without cancelling the run.

Actual behavior

• The queued-steer card says esc cancel without identifying the run as the cancellation target.
• Escape sends a run Cancel frame and deliberately sends no steer_cancel frame.
• The pending steer is recorded at terminal close and emitted as a steer echo.
• Mecatui renders that echo as a user message with steer applied, but the cancelled run does not process it.

Environment (if relevant)

• mecatl commit: ad1cfe3c89a640905b88fb69f9905df498ba5c9c (origin/main on 2026-09-18)
• Deployment shape: mecatui

Additional context

Current tests explicitly pin both sides of this behavior:

• cmd/mecatui/ui/steer_test.go: esc cancels without retracting a pending steer
• engine/agent/steer_test.go: TestSteer_NeverClosedParked records the steer at cancellation so a future run can replay it

The card changed from esc retract to esc cancel when direct run cancellation was introduced in #887.

Focused verification:

go test ./cmd/mecatui/ui -run TestSteer_TUIRendersAuthoritativeState -count=1
cd engine && go test ./agent -run TestSteer_NeverClosedParked -count=1

Start with cmd/mecatui/ui/steer_test.go and engine/agent/steer_test.go, then trace Escape handling and terminal-close history recording in the mecatui and agent steer paths. Clarify whether Escape retracts the steer or cancels the run, and update the card and history status consistently. Run the two focused Go tests named in the issue, adding coverage for the chosen behavior.