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/schema/json/codex_app_server_protocol.schemas.json

@@ -7540,6 +7540,12 @@
       },
       "ConfigRequirements": {
         "properties": {
+          "allowManagedHooksOnly": {
+            "type": [
+              "boolean",
+              "null"
+            ]
+          },
           "allowedApprovalPolicies": {
             "items": {
               "$ref": "#/definitions/v2/AskForApproval"