Contribution proposal: scope /btw Escape cancellation to in-flight side queries

## What do you want to change?

Refine `/btw` Escape ownership by side-query state:

- While side work is in flight, Escape dismisses the panel, aborts that side request, and is consumed so the concurrent main turn continues.
- After the side query settles, Escape dismisses its panel but passes through so the existing main-turn interrupt remains available.

## Why?

PR #261 intentionally chose “one Escape cancels both.” In practice, that makes one gesture cancel two independent operations: the visible `/btw` side query and the unrelated main turn. The side panel should own Escape only while it still has cancellable work.

Repro: start a slow main turn, run `/btw `, then press Escape while both are active. Today both requests abort; the proposed behavior aborts only `/btw`.

## How?

Keep the change inside the builtin BTW terminal listener. Consume Escape only for an unsettled entry; dismiss and return `undefined` after settlement. Add deterministic regressions for both states and real-TUI QA for main continuation, settled pass-through, follow-up input, and credential isolation.

A verified branch is ready at https://github.com/honb0704/senpi/tree/fix/btw-escape-consume--omo-01a00dd6. It passes the 17-test BTW suite, `npm run check`, and five independent review lanes. Open PR #880 does not supersede the listener change, though its BTW documentation/test imports may need conflict resolution.

Start in the builtin BTW terminal listener and inspect how Escape is handled for unsettled versus settled side queries; the issue does not name a file path. Run the 17-test BTW suite and npm run check, then perform the real-TUI checks for main continuation, settled pass-through, follow-up input, and credential isolation.

Written by the indexing model from the issue text.