[Feature]: Let a provider/account switch take effect mid-run (at the next tool boundary), not only when the turn ends

[a2mw]

Area

apps/web, apps/mobile, orchestration

Problem

With several accounts of one harness linked (e.g. three Codex accounts), I sometimes need to move a running agent or subagent to another account to manage usage, so it doesn't run out and stop fully. Picking another account in the composer shows "Handing off", but the switch only applies when the next turn starts.

For a subagent, the current turn is usually the whole task. So "Handing off" just sits there until the work is finished on the original account, which defeats the point.

The only way to force it today is to interrupt the subagent and send something like "Continue". That throws away the in-flight step, and the parent agent is told that its child stopped without finishing (causes issues because the parent agent does not know why and attributes it to some problem, trying to then restart the agent by creating a new thread or also sending messages to this one).

Related: #25261, which would effectively manage doing this automatically, but it's a bigger change in scope, because it would need usage-limit decisioning.

Requested behaviour

Make it configurable when a pending handoff applies:

• At the next tool boundary: once the current tool call finishes, end the provider turn there and continue the same run on the new account with the handoff context. No user interrupt, and the parent sees no stop or failure.
• At turn end: today's behaviour.

Version

Desktop 0.1.5 (macOS), iOS app.