Address review feedback from cross-vendor review (Flash + GPT-OSS):
1. Auto-detect now checks for headless Linux (no DISPLAY), WSL2
(/proc/version contains 'microsoft'), instead of all Linux.
Desktop Linux with a compositor keeps the overlay.
2. Add _cua_driver_supports_no_overlay() that probes cua-driver --help
to check if the flag is supported. Older drivers (< 0.6.x) reject
unknown flags, so passing --no-overlay would crash the MCP spawn.
3. Update tests to cover headless vs desktop Linux, WSL2 detection,
version probe, and the unsupported-driver fallback path.
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