fix: resolve Tokio panic and silence logs in SSH askpass mode

[Cleboost]

This pull request refactors the startup and AskPass handling logic in src/main.rs to improve clarity and reliability. The main changes are focused on how logging is initialized and how the AskPass password retrieval flow is handled asynchronously.

Improvements to logging and initialization:

• Logging initialization is now skipped when running in AskPass mode (when the RUSTMIUS_ASKPASS_ALIAS environment variable is set), preventing unnecessary log output during password retrieval.
• The tracing subscriber now explicitly writes logs to stderr for better separation of logs and program output.

AskPass flow improvements:

• The AskPass password retrieval now uses async/await directly instead of blocking on the runtime, making the code simpler and more idiomatic.
• Password output to stdout is now performed using write_all and flush to ensure the password is reliably sent to the caller (such as SSH).
• Redundant debug and error logging during AskPass handling has been removed to avoid interfering with the expected output.

fix #146