feat(config): support managed deny-read requirements (#17740)

## Summary
- adds managed requirements support for deny-read filesystem entries
- constrains config layers so managed deny-read requirements cannot be
widened by user-controlled config
- surfaces managed deny-read requirements through debug/config plumbing

This PR lets managed requirements inject deny-read filesystem
constraints into the effective filesystem sandbox policy.
User-controlled config can still choose the surrounding permission
profile, but it cannot remove or weaken the managed deny-read entries.

## Managed deny-read shape
A managed requirements file can declare exact paths and glob patterns
under `[permissions.filesystem]`:

```toml
# /etc/codex/requirements.toml
[permissions.filesystem]
deny_read = [
  "/Users/alice/.gitconfig",
  "/Users/alice/.ssh",
  "./managed-private/**/*.env",
]
```

Those entries are compiled into the effective filesystem policy as
`access = none` rules, equivalent in shape to filesystem permission
entries like:

```toml
[permissions.workspace.filesystem]
"/Users/alice/.gitconfig" = "none"
"/Users/alice/.ssh" = "none"
"/absolute/path/to/managed-private/**/*.env" = "none"
```

The important difference is that the managed entries come from
requirements, so lower-precedence user config cannot remove them or make
those paths readable again.

Relative managed `deny_read` entries are resolved relative to the
directory containing the managed requirements file. Glob entries keep
their glob suffix after the non-glob prefix is normalized.

## Runtime behavior
- Managed `deny_read` entries are appended to the effective
`FileSystemSandboxPolicy` after the selected permission profile is
resolved.
- Exact paths become `FileSystemPath::Path { access: None }`; glob
patterns become `FileSystemPath::GlobPattern { access: None }`.
- When managed deny-read entries are present, `sandbox_mode` is
constrained to `read-only` or `workspace-write`; `danger-full-access`
and `external-sandbox` cannot silently bypass the managed read-deny
policy.
- On Windows, the managed deny-read policy is enforced for direct file
tools, but shell subprocess reads are not sandboxed yet, so startup
emits a warning for that platform.
- `/debug-config` shows the effective managed requirement as
`permissions.filesystem.deny_read` with its source.

## Stack
1. #15979 - glob deny-read policy/config/direct-tool support
2. #18096 - macOS and Linux sandbox enforcement
3. This PR - managed deny-read requirements

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server/src/config_api.rs

@@ -605,6 +605,7 @@ mod tests {
                 }),
                 allow_local_binding: Some(true),
             }),
+            permissions: None,
         };
 
         let mapped = map_requirements_toml_to_api(requirements);
@@ -698,6 +699,7 @@ mod tests {
                 }),
                 allow_local_binding: None,
             }),
+            permissions: None,
         };
 
         let mapped = map_requirements_toml_to_api(requirements);
@@ -739,6 +741,7 @@ mod tests {
             rules: None,
             enforce_residency: None,
             network: None,
+            permissions: None,
         };
 
         let mapped = map_requirements_toml_to_api(requirements);