The `cli_default_open_behavior` setting controls how `zed <path>` opens
directories, but the underlying behavior is gated behind the
`AgentV2FeatureFlag` in `open_listener.rs`. Without the flag enabled,
the setting has no effect — Zed always falls back to `NewWindow`
behavior.

This change gates the settings UI entry behind the same feature flag so
it only appears when the feature is actually active.

**Changes:**
- Converted `general_settings_section()` from a fixed-size array to a
`Vec` so the CLI setting can be conditionally appended
- Added `feature_flags` as a dependency to `settings_ui`

Release Notes:

- N/A