Prewarm shell snapshots for eligible turns

[copyberry]

Prewarm shell snapshots for eligible turns

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.