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.v2.schemas.json

@@ -3929,6 +3929,12 @@
     },
     "ConfigRequirements": {
       "properties": {
+        "allowManagedHooksOnly": {
+          "type": [
+            "boolean",
+            "null"
+          ]
+        },
         "allowedApprovalPolicies": {
           "items": {
             "$ref": "#/definitions/AskForApproval"