## Why
Credential-brokered commands rebuild shell snapshots on every invocation. Snapshot capture failures can also include credential-bearing startup output, and descendants holding output pipes need cleanup even after the shell exits.
## What changed
- Cache successful protected snapshots per environment, keyed by working directory, shell, login mode, and sandbox configuration. Rebuild missing or stale snapshots while keeping concurrent captures independently cancellable.
- Invalidate snapshots when shell settings or credential broker configuration change, and retry capture once if broker configuration changes during startup.
- Restore credentials and associated provider context while respecting environment policy filters and explicit overrides.
- Keep full-buffer capture subject to timeout and cancellation through output draining, clean up descendants on capture expiration or drain failure, and omit startup output from snapshot errors.
## Testing
Add regression coverage for snapshot reuse, invalidation, recovery after storage failure, concurrent cancellation, credential-safe errors, and descendant cleanup. Verify that successful background startup and output beyond the shell output cap remain supported.
GitOrigin-RevId: 7c7c8455bf6e3b9ca173dbb4277783469f0b45e6
Winston Howes
808b3411fd