Isolate client files when copying VPS profiles

[mtmtian]

Summary:

• reset CLIENT_FILE_PREFIX to the destination profile when copying deploy.conf
• report the actual configured client file prefix after deployment
• use the existing CStoneCloud profile name in migration examples
• add regressions proving source config, secrets, and existing client filenames stay isolated

Validation:

• shell syntax checks passed
• 32 Python tests passed; 1 Mihomo integration test skipped because the binary is unavailable
• no profile or client credential files are tracked