## Why
Executor config reads omitted the startup `features.prefer_mxc` override, so clients could not see the CLI sandbox preference.
## What changed
- Include the explicit CLI `features.prefer_mxc` value in `environmentConfig/read` as a `session-flags` layer, above project config and below legacy managed file and MDM config.
- Retain only this preference from startup overrides.
- Expose `windows_mxc_allowed_by_config` to share MXC eligibility checks after callers resolve the active permission profile.
## Testing
Add CLI and remote executor coverage for preference propagation and filtering, plus a unit test verifying that both legacy managed config sources retain precedence.
GitOrigin-RevId: 5ac6f5257f1e807c2f197dcf5dcb2385cedca0ea