Problem: the `:help oil-adapter-ssh` section was only 10 lines covering
URL format and server requirements. Users had no guidance on how the
adapter works, how to configure `extra_scp_args`, or how to troubleshoot
connection issues.
Solution: expand the section with architecture overview (persistent SSH
connection, `scp` for file transfer, no local mount), configuration
(`extra_scp_args`, `~/.ssh/config`), limitations (no third-party plugin
integration, no cross-adapter moves), and troubleshooting tips.
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