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/core/src/rollout/metadata.rs

@@ -49,7 +49,7 @@ pub(crate) fn builder_from_session_meta(
     builder.model_provider = session_meta.meta.model_provider.clone();
     builder.cwd = session_meta.meta.cwd.clone();
     builder.cli_version = Some(session_meta.meta.cli_version.clone());
-    builder.sandbox_policy = SandboxPolicy::ReadOnly;
+    builder.sandbox_policy = SandboxPolicy::new_read_only_policy();
     builder.approval_mode = AskForApproval::OnRequest;
     if let Some(git) = session_meta.git.as_ref() {
         builder.git_sha = git.commit_hash.clone();