Alias codex_hooks feature as hooks (#20522)

# Why

The hooks feature flag should use the concise canonical name `hooks`,
while existing configs that still use `codex_hooks` continue to work
during the rename.

# What

- change the canonical `Feature::CodexHooks` key from `codex_hooks` to
`hooks`
- register `codex_hooks` through the existing legacy-alias path
- update the config schema and canonical config fixtures to prefer
`hooks`
- add regression coverage that both `hooks` and `codex_hooks` resolve to
`Feature::CodexHooks`

# Verification

- `cargo test -p codex-features`
- `cargo test -p codex-core config::schema_tests`
- `cargo test -p codex-core
pre_tool_use_blocks_shell_when_defined_in_config_toml`
- `cargo test -p codex-app-server
hooks_list_uses_each_cwds_effective_feature_enablement`

codex-rs/features/src/tests.rs

@@ -267,6 +267,12 @@ fn collab_is_legacy_alias_for_multi_agent() {
     assert_eq!(feature_for_key("collab"), Some(Feature::Collab));
 }
 
+#[test]
+fn codex_hooks_is_legacy_alias_for_hooks() {
+    assert_eq!(feature_for_key("hooks"), Some(Feature::CodexHooks));
+    assert_eq!(feature_for_key("codex_hooks"), Some(Feature::CodexHooks));
+}
+
 #[test]
 fn multi_agent_is_stable_and_enabled_by_default() {
     assert_eq!(Feature::Collab.stage(), Stage::Stable);