feat: make sandbox read access configurable with ReadOnlyAccess (#11387)

`SandboxPolicy::ReadOnly` previously implied broad read access and could
not express a narrower read surface.
This change introduces an explicit read-access model so we can support
user-configurable read restrictions in follow-up work, while preserving
current behavior today.

It also ensures unsupported backends fail closed for restricted-read
policies instead of silently granting broader access than intended.

## What

- Added `ReadOnlyAccess` in protocol with:
  - `Restricted { include_platform_defaults, readable_roots }`
  - `FullAccess`
- Updated `SandboxPolicy` to carry read-access configuration:
  - `ReadOnly { access: ReadOnlyAccess }`
  - `WorkspaceWrite { ..., read_only_access: ReadOnlyAccess }`
- Preserved existing behavior by defaulting current construction paths
to `ReadOnlyAccess::FullAccess`.
- Threaded the new fields through sandbox policy consumers and call
sites across `core`, `tui`, `linux-sandbox`, `windows-sandbox`, and
related tests.
- Updated Seatbelt policy generation to honor restricted read roots by
emitting scoped read rules when full read access is not granted.
- Added fail-closed behavior on Linux and Windows backends when
restricted read access is requested but not yet implemented there
(`UnsupportedOperation`).
- Regenerated app-server protocol schema and TypeScript artifacts,
including `ReadOnlyAccess`.

## Compatibility / rollout

- Runtime behavior remains unchanged by default (`FullAccess`).
- API/schema changes are in place so future config wiring can enable
restricted read access without another policy-shape migration.

codex-rs/tui/src/debug_config.rs

@@ -474,13 +474,14 @@ mod tests {
         } else {
             absolute_path("/etc/codex/requirements.toml")
         };
+
         let requirements = ConfigRequirements {
             approval_policy: ConstrainedWithSource::new(
                 Constrained::allow_any(AskForApproval::OnRequest),
                 Some(RequirementSource::CloudRequirements),
             ),
             sandbox_policy: ConstrainedWithSource::new(
-                Constrained::allow_any(SandboxPolicy::ReadOnly),
+                Constrained::allow_any(SandboxPolicy::new_read_only_policy()),
                 Some(RequirementSource::SystemRequirementsToml {
                     file: requirements_file.clone(),
                 }),
@@ -572,7 +573,6 @@ mod tests {
         ));
         assert!(!rendered.contains("  - rules:"));
     }
-
     #[test]
     fn debug_config_output_lists_session_flag_key_value_pairs() {
         let session_flags = toml::from_str::<TomlValue>(