Add allow_managed_hooks_only hook requirement (#20319)

## Why

Enterprise-managed hook policy needs a narrow way to require Codex to
ignore user-controlled lifecycle hooks without adopting the broader
trust-precedence model from earlier hook work. This keeps the policy
anchored in `requirements.toml`, so admins can opt into managed hooks
only while normal `config.toml` files cannot enable the restriction
themselves.

## What changed

- Added `allow_managed_hooks_only` to the requirements data flow and
preserved explicit `false` values.
- Also adds it to /debug-config
- Marked MDM, system, and legacy managed config layers as managed for
hook discovery.
- Updated hook discovery so `allow_managed_hooks_only = true`:
  - keeps managed requirements hooks and managed config-layer hooks,
- skips user/project/session `hooks.json` and `[hooks]` entries with
concise startup warnings,
  - skips current unmanaged plugin hooks,
- ignores any `allow_managed_hooks_only` key placed in ordinary
`config.toml` layers.

codex-rs/app-server/src/request_processors/config_processor.rs

@@ -429,6 +429,7 @@ fn map_requirements_toml_to_api(requirements: ConfigRequirementsToml) -> ConfigR
             }
             normalized
         }),
+        allow_managed_hooks_only: requirements.allow_managed_hooks_only,
         feature_requirements: requirements
             .feature_requirements
             .map(|requirements| requirements.entries),
@@ -611,3 +612,21 @@ fn config_write_error(code: ConfigWriteErrorCode, message: impl Into<String>) ->
     }));
     error
 }
+
+#[cfg(test)]
+mod tests {
+    use super::map_requirements_toml_to_api;
+    use codex_config::ConfigRequirementsToml;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn requirements_api_includes_allow_managed_hooks_only() {
+        let mapped = map_requirements_toml_to_api(ConfigRequirementsToml {
+            allow_managed_hooks_only: Some(true),
+            ..ConfigRequirementsToml::default()
+        });
+
+        assert_eq!(mapped.allow_managed_hooks_only, Some(true));
+        assert_eq!(mapped.hooks, None);
+    }
+}