SSH clone is native-only: document or handle gracefully on JS target

[mizchi]

Context

PR #17 implemented SSH clone support using the native interactive protocol with pipe-based I/O (spawn_orphan + ReadFromProcess/WriteToProcess).

This fix is inherently native-only because:

• SSH transport spawns ssh as a subprocess
• Pipe-based I/O (@process.spawn_orphan, @process.write_to_process, @process.read_from_process) is only available on native targets
• The implementation lives in src/io/native/upload_pack_process.mbt

Current behavior

On JS target, bit clone is not supported at all — any non-relay command returns "bit: '<cmd>' is not supported in JS target".

Considerations

• Document that SSH clone requires the native binary
• Consider whether JS target should support clone over HTTP (smart HTTP protocol doesn't need subprocess pipes)
• The relay-based clone (bit clone relay+https://...) works on both targets via HTTP