Document CStoneCloud panel handoff

[mtmtian]

Summary:

• record the verified CStoneCloud lifecycle and SSH-key controls
• prefer panel key binding and keep interactive key installation as fallback
• add plaintext-password and post-reinstall key verification safeguards

Validation:

• bash syntax checks passed
• 32 Python tests passed; 1 Mihomo integration test skipped because the binary is unavailable
• no profile state is tracked