Refactor CLI with System.CommandLine & SSH key support

[brian-shelledy]

• Refactored the application to use System.CommandLine for argument parsing, replacing manual parsing logic.
• Added support for SSH private key authentication (id_rsa) in addition to password authentication.
• Updated launchSettings.json to use options for password and added new test profiles.
• Made password parameter nullable and centralized SftpClient creation logic.
• Updated Program.cs for async command invocation.

These changes improve CLI usability, authentication flexibility, and testability.

[SteveIves]

Thanks for the contribution. The overall direction—moving to System.CommandLine and adding SSH private-key authentication—is useful. I reviewed and built the changes, but I found several issues that I think need to be addressed before this is ready to merge.

1. -h conflicts with the built-in help alias.
   SFTPSync/SFTPSync.cs:13 assigns -h to --host, while System.CommandLine also registers -h for help. Running with -h dummy displays help instead of starting the application. The new launch profile also uses this broken syntax. Please use another host alias such as -H, or omit the short alias.

2. Contributor-specific settings are packaged and loaded automatically.
   SFTPSync/sftpsyncsettings.json contains environment-specific host, username, local-path, and remote-path values. SFTPSync.csproj copies that file into the output, and Program.cs loads it by default. On a machine where those paths and authentication are available, simply starting the CLI without arguments could attempt that sync. This should be an empty/example configuration and preferably loaded only when explicitly requested.

3. Existing UI configuration passwords will not work.
   SFTPSync/SFTPSync.cs:67 reads RemotePassword directly, but the UI encrypts that field with DPAPI before saving it and decrypts it before creating an SFTP connection (SFTPSyncUI/SettingsForm.cs:229 and SFTPSyncUI/SFTPSyncUI.cs:283). Passing an existing UI settings file to the CLI would therefore send the encrypted Base64 value as the password.

4. The incremental file-update code changes file contents.
   In SFTPSyncLib/RemoteSync.cs:232-241, ReadLine() removes each line terminator, so the subsequent Replace() calls cannot normalize it. WriteLine() then writes the Windows newline sequence and adds a final newline. Incremental copies may therefore differ from initial copies. The code also deletes the existing remote file before opening the replacement, which risks losing the remote copy and its metadata if writing fails. Please preserve the original bytes, or define and test an explicit conversion policy and use a safer replacement strategy.

5. A missing search pattern reports a successful no-op.
   searchPattern is not validated. The null value becomes an empty worker loop, after which --one-time logs Sync Complete and exits successfully without syncing anything. I reproduced this locally. It should be required from either the command line or configuration and should return an error when missing or empty.

6. The existing documented CLI is broken, and parse failures return exit code 0.
   The current documentation still specifies six positional arguments, but all of them are rejected by the new options-only parser. If this is an intentional breaking change, the documentation and examples need to be updated; ideally the old form should remain compatible for a transition period. Also, Program.cs:32 discards the integer returned by InvokeAsync(), so my rejected legacy invocation still exited with code 0. The entry point should propagate it, for example:

   return await parseResult.InvokeAsync();

7. The submodule URL was changed from HTTPS to SSH.
   .gitmodules now uses git@github.com:..., which makes normal public submodule initialization depend on the user having GitHub SSH authentication configured. Unless there is a specific requirement for SSH, the HTTPS URL is more appropriate for a public dependency.

The project builds successfully for me with zero warnings and errors. I also exercised --help, short-host parsing, legacy positional parsing, and the missing-pattern case without making an SFTP connection. There are currently no PR checks or application-level tests covering these new parser, configuration, and authentication behaviors.

I would like to see the issues above—particularly items 1 through 6—resolved before merging.