feat(cli): add /btw for one-shot side questions

[lit26]

What

/btw <question> asks a quick question without disturbing the main conversation. An overlay opens, answers, and closes on esc — nothing is appended to the transcript, the session file, or the model's context.

Why this shape

The obvious implementations both fail:

• Send it as a normal turn — the reply is an ordinary assistant message, and interactive-mode.ts renders every one of those unconditionally. The answer appears twice: once in the overlay, once below it.
• Hide it at render time — a global "skip the next assistant message" flag in front of the transcript fails as "your real answer vanished", which is worse than a duplicate.

So the side question never enters the agent loop at all. It goes through modelRegistry.complete(), which talks to the same provider with the same credentials but owns no agent and no session. There is nothing to append and nothing to rewind, a main turn in flight is untouched, and tools are unreachable because complete() has no tool loop.

Details worth a second look

• Message fork. The question is sent against a copy of the current branch (buildContextEntries → convertToLlm), with a trailing assistant message holding unanswered tool calls dropped. That is exactly the state a streaming main turn leaves behind, and providers that validate message order reject it.
• History. Prior exchanges live in an in-memory BtwHistory, capped at 20, and are replayed only into the next side question. Main turns never see them. Deliberately not persisted: writing it down would put the exchanges back into the session this command exists to keep them out of.
• Spinner. Reuses pi-tui's Loader as a frame clock only — the frame is read out of its paint callback and the panel draws its own row, matching view/transcript/bash-execution.ts. The interval is released on both finish() and dispose(); the overlay host already calls dispose() on close.

Panel

esc close · c copy · x clear history · shift+←/→ browse earlier exchanges · ↑/↓ scroll. Shows (+N earlier /btw) when history is non-empty.

Tests

Three cases in test/step-slash-commands.test.ts (30 passing in that file):

• answers out of band — asserts sendMessage is never called, the dangling tool call is stripped from the fork, the answer reaches the panel, the next side question carries the previous answer, and x clears it
• spinner advances on the Loader clock, and finish() + dispose() stop requesting renders (no leaked timer per /btw)
• rejects an empty question and a session with no model

pnpm check passes (biome + all 16 repo invariant checks + tsgo --noEmit).

Known gaps

• No token streaming. complete() resolves with the finished message, so the panel shows the spinner and then the whole answer. Streaming would need stream() exposed on the ModelRegistry facade.
• Not exercised in a live TUI — behaviour is derived from the overlay host contract (showExtensionCustom calls component.dispose() on close) rather than observed end to end.