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/linux-sandbox/src/bwrap.rs

@@ -141,6 +141,13 @@ fn create_bwrap_flags(
 /// 4. `--dev-bind /dev/null /dev/null` preserves the common sink even under a
 ///    read-only root.
 fn create_filesystem_args(sandbox_policy: &SandboxPolicy, cwd: &Path) -> Result<Vec<String>> {
+    if !sandbox_policy.has_full_disk_read_access() {
+        return Err(CodexErr::UnsupportedOperation(
+            "Restricted read-only access is not yet supported by the Linux bubblewrap backend."
+                .to_string(),
+        ));
+    }
+
     let writable_roots = sandbox_policy.get_writable_roots_with_cwd(cwd);
     ensure_mount_targets_exist(&writable_roots)?;