feat(native): add SFTP client API

[lovewave02]

Summary

• add a native SFTPClient opened through SSHClient.open_sftp()
• expose cwd-aware directory, metadata, mutation, and transfer operations
• keep existing SSHClient SFTP methods compatible and document native-only support

Verification

• focused native SFTP unit and integration tests: 9 passed
• package unit tests: 29 passed
• flake8 pssh tests ci/integration_tests
• RST check and Sphinx HTML build
• existing native integration failures compared against origin/master: no new failures

Closes #410.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.54%. Comparing base (97a7414) to head (18c4db7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #426      +/-   ##
==========================================
+ Coverage   99.53%   99.54%   +0.01%     
==========================================
  Files          18       19       +1     
  Lines        1703     1758      +55     
==========================================
+ Hits         1695     1750      +55     
  Misses          8        8              

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