Claude CLI backend defaults preserved on registry absence

The Claude CLI backend was failing to bootstrap when the plugin registry was absent. Specifically, the configuration resolution logic would return null instead of falling back to default settings, breaking workspace bootstrap file injection (AGENTS.md, SOUL.md, IDENTITY.md, USER.md) on first turns. Additionally, environment variable overrides specified in backend configs weren't being applied correctly to spawned processes—sanitization was using the wrong base environment.

These fixes ensure the backend operates correctly in minimal environments where plugin registries aren't available. For operators running isolated or air-gapped setups, bootstrap files will now be injected as expected, and environment variable configurations will take effect properly.

The changes span the CLI backend configuration resolution, the process spawning logic, and gateway server lifecycle management. A live probe test was added to deterministically verify bootstrap injection works in real subprocess scenarios.