fix(config): use deny for Unix socket permissions (#24970)

## Why

Unix socket permissions still accepted and displayed `"none"` while file
permissions use the clearer `"deny"` spelling. This keeps network Unix
socket policy vocabulary consistent with filesystem policy vocabulary.

## What changed

- Replace the Unix socket permission variant and serialized spelling
from `none` to `deny` across config, feature configuration, and network
proxy types.
- Update app-server v2 serialization, TUI debug output, focused tests,
and generated schemas to expose `"deny"`.
- Add coverage for denied Unix socket entries in managed requirements
and profile overlay behavior.

## Security

This is a vocabulary change for explicit Unix socket rejection, not a
network access expansion. Denied entries continue to be omitted from the
effective allowlist.

## Validation

- `just fmt`
- `just write-config-schema`
- `just write-app-server-schema`
- `just test -p codex-config -p codex-core -p codex-app-server-protocol
-p codex-tui -E
'test(network_requirements_are_preserved_as_constraints_with_source) |
test(network_permission_containers_project_allowed_and_denied_entries) |
test(network_toml_overlays_unix_socket_permissions_by_path) |
test(permissions_profiles_resolve_extends_parent_first_with_child_overrides)
| test(network_requirements_serializes_canonical_and_legacy_fields) |
test(debug_config_output_formats_unix_socket_permissions)'`\n- Automatic
`bench-smoke` follow-up from `just test`\n- `cargo clippy -p
codex-config -p codex-core -p codex-features -p codex-network-proxy -p
codex-app-server-protocol -p codex-app-server -p codex-tui --all-targets
-- -D warnings`

codex-rs/network-proxy/src/config.rs

@@ -107,7 +107,7 @@ impl NetworkDomainPermissions {
 #[serde(rename_all = "lowercase")]
 pub enum NetworkUnixSocketPermission {
     Allow,
-    None,
+    Deny,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, Default, PartialEq, Eq)]