PAC 1 - Add system proxy feature config surface (#26706)

## Summary

Introduces the default-off `respect_system_proxy` feature flag used to
gate first-class system PAC/proxy support for Codex-owned native
clients.

With the feature disabled or absent, behavior remains unchanged. This PR
establishes the configuration and managed-requirement surface; proxy
discovery and request routing are implemented by follow-up PRs.

## Configuration

User configuration uses the standard boolean feature form:

```toml
[features]
respect_system_proxy = true
```

Managed feature requirements use the corresponding boolean key. The
effective runtime configuration is exposed as a boolean and defaults to
`false`.

## Implementation

- Registers `respect_system_proxy` as an under-development, default-off
feature.
- Resolves user configuration and managed feature requirements into
`Config.respect_system_proxy`.
- Provides bootstrap resolution for startup paths that must evaluate the
feature before full configuration loading completes.
- Uses the standard feature CLI and config-editing behavior.
- Excludes `features.respect_system_proxy` from project-local
configuration.
- Updates the generated configuration schema.

## End-user behavior

- No networking behavior changes when the feature is absent or disabled.
- Enabling the feature makes the boolean available to the native
proxy-routing implementation in follow-up PRs.
- Repository-local configuration cannot enable the feature.

## Test coverage

Covers scalar configuration and CLI override resolution, managed
requirement constraints, bootstrap resolution, and project-local
filtering.

codex-rs/features/src/tests.rs

@@ -649,6 +649,7 @@ fn materialize_resolved_enabled_writes_all_features_and_preserves_custom_config(
     features.enable(Feature::CodeMode);
     features.enable(Feature::MultiAgentV2);
     features.enable(Feature::NetworkProxy);
+    features.enable(Feature::RespectSystemProxy);
 
     let mut features_toml = FeaturesToml {
         multi_agent_v2: Some(FeatureToml::Config(crate::MultiAgentV2ConfigToml {