feat(coding-agent): let an extension run the turn

[moedash]

What changed?

pi.registerTurnExecutor(executor, options?). Pi hands the turn to the executor instead of running it, and the executor decides when to call turn.run(). run() is the same turn pi would have run, so the transcript, the events and the streaming do not change. An executor that returns without calling it holds the turn.

With resumeOnStart, opening a session also hands the executor a turn an earlier run left unfinished, which routes through resumeInterruptedTurn() and so through the executor like any other turn.

One executor wins, the first registration. None registered leaves pi running turns itself, which is every existing session.

Stacked on #2, which added prepareStep() and resumeInterruptedTurn().

Why?

Nothing outside pi can decide when a turn happens. prompt() runs the turn the moment it accepts the text, and an extension has no handle on the AgentSession to drive it instead: ExtensionContext gives a read-only session manager. So an extension that wants turns to go through a scheduler, or a durable executor, has to intercept input and run the work in a session of its own, which leaves the transcript you are looking at empty.

This is the seam that avoids that. The executor runs in this process against the live session, so what the user sees is pi's own turn.

resumeOnStart is the other half. A crash during a tool call leaves a call with no result and pi does not finish it, so the durability only shows up if something picks that turn up when the session opens again.

How did you test it?

• Unit Tests
• Staging
• End to End Tests

npm run check passes. New tests cover the default (no executor registered), first-registration-wins, a wrapped turn producing the same transcript as an unwrapped one, a held turn staying unrun, resume-on-start finishing an interrupted turn without re-prompting, and a finished session being left alone.