fix: post OMP ask questions to the room instead of aborting the turn

[bandzalkin]

Stacked on #763. The native OMP adapter exists only in #763, so this diff includes #763's commits until it merges. This PR's change is the top commit, feat(omp): route the native ask tool to the room (de52cbf8).

Why

Every OMP turn that used OMP's built-in ask tool was aborted. Without the structured ask dialog, ask reaches the host as select/editor UI requests, which OmpAdapter can only cancel. OMP treats a cancelled ask as the user interrupting the turn (prompt_result: aborted), so the room got "Internal error while processing message" and the message was marked failed. This happened in production when a model called ask alongside band_send_message.

What

ask is now answered through the room, using the same turn split as Copilot's ask_user="room". Band processes a room's messages one at a time, so the turn cannot block waiting for the answer.

• Opt in per session. _open_room sends set_ask_dialog {enabled: true} and fails startup if OMP refuses. OMP ≥ 18.8.9, the documented minimum, supports it.
• Post and acknowledge. An ask request goes to OmpAdapter._ask_room. It posts the questions, options, descriptions and recommendation to the room through deliver_reply, mentioning the requester, and that post is the turn's reply. Each question is answered immediately with customInput text telling the model the answer arrives as a later message. The participant's reply resumes the same native session.
• Failure paths. If the room post fails, the model is told so and the turn still owes a reply. A stopped room (RoomExecutionStoppedError) still ends the turn.
• Wire models. Adds OmpAskQuestion, OmpAskOption and OmpAskAnswer, plus OmpUIRequest.questions and OmpUIResponse.answers. recommended accepts any number, matching OMP's schema.
• Shutdown guard. Once shutdown begins, OmpRpcClient._start_callback refuses new host-tool and UI callbacks. Before, a callback arriving after cleanup ran with no owner and could still post to the room.
• Guidance and docs. The system prompt explains the ask turn split, and docs/acp.md documents the behavior.

Scope

• When a host tool (_execute_tool, from fix(integrations): preserve native OMP lifecycle and prompt authority #763) raises an unscoped RoomExecutionStoppedError, the model still gets an error result and the turn does not end. This PR doesn't change that.
• ask calls from OMP subagents get no special handling.