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-protocol/src/protocol/v2/config.rs

@@ -357,6 +357,7 @@ pub struct ConfigRequirements {
     pub allowed_approvals_reviewers: Option<Vec<ApprovalsReviewer>>,
     pub allowed_sandbox_modes: Option<Vec<SandboxMode>>,
     pub allowed_web_search_modes: Option<Vec<WebSearchMode>>,
+    pub allow_managed_hooks_only: Option<bool>,
     pub feature_requirements: Option<BTreeMap<String, bool>>,
     #[experimental("configRequirements/read.hooks")]
     pub hooks: Option<ManagedHooksRequirements>,

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -1688,6 +1688,7 @@ fn config_requirements_granular_allowed_approval_policy_is_marked_experimental()
             allowed_approvals_reviewers: None,
             allowed_sandbox_modes: None,
             allowed_web_search_modes: None,
+            allow_managed_hooks_only: None,
             feature_requirements: None,
             hooks: None,
             enforce_residency: None,