[FEATURE]: pluggable session execution for crash-resumable sessions

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Today a session lives and dies with the serving process. If the daemon or host dies mid-turn, the in-flight turn is lost; a session can't resume on another machine, and a long background run needs the process to stay alive for hours.

The v2 engine already concentrates execution behind a small abstraction: SessionExecution is four methods (active / wake / resume / interrupt), and the turn work happens in drains behind it. That abstraction is nearly a driver contract already.

The request is to let a config or env switch pick a session-execution driver, keep the in-process driver as the default, and define the contract so an external orchestrator can supply durability: crash resume mid-turn, resume on a different worker from shared storage, detached long runs.

We (@temporalio) built a working proof on a fork (https://github.com/temporalio/opencode/pull/2): the session supervisor is written once and runs under two drivers, in-process by default and as a Temporal workflow with one activity per step. A session survives kill -9 of the worker mid-turn and completes on a fresh worker; worktrees rebuild from snapshot packs in the shared store. We'd rather upstream this (and optionally a reference driver) than carry a fork, and we're happy to change it to whatever contract you'd accept.