## Why
Shell Snapshot V2 previously captured the login-shell environment lazily when a
command started, adding that work to the command path.
## What changed
- Start one asynchronous snapshot capture after turn hooks accept the turn, so
eligible local `exec_command` calls can reuse it without blocking the model.
- Use the turn's resolved tools, permissions, sandbox, environment, and shell
configuration, and skip prewarming for unsupported or network-managed cases.
- Cancel outstanding prewarming during shutdown. Keep failed speculative
captures retryable by real commands without consuming their retry budget.
## Testing
Added coverage for non-blocking capture and reuse, failure recovery, sandbox and
permission isolation, shutdown cancellation, hook-stopped turns, eligibility
gates, and concurrent capture retries.
GitOrigin-RevId: ff24a4f0328c06653e09d03fd0a3db1eccd65030
612e6491d5