The card was macOS-only. cua-driver also runs on Windows and Linux, so
fold `cua-driver doctor` (cross-platform binary/health probes) into a
single OS-aware `ready` signal:
- macOS: ready == both TCC grants; keeps the permission rows + grant flow.
- Windows/Linux: no TCC toggles, so ready == driver health, with a
per-OS note (SmartScreen/UIAccess on Windows; X11/XWayland on Linux).
`computer_use_status()` replaces the macOS-only `permissions_status()` and
surfaces `platform`, `ready`, `can_grant`, and the doctor `checks` (non-ok
ones render as warnings). CLI `permissions status`, the REST endpoint, and
the desktop card all key off the one payload. Grant stays macOS-only (400
elsewhere — nothing to grant).
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