Log generated config output path and document systemd regeneration flow

[rainbend]

Summary

• Log the generated config destination after a successful run, using an absolute path for file outputs and stdout when writing to standard output.
• Add a regression test to verify the output path is included in CLI stderr.
• Document a systemd ExecStartPre pattern for regenerating sing-box config before service startup in both English and Chinese README files.

Testing

• Added CLI test coverage for the emitted output-path log line.
• Not run (not requested).