feat(sftp): tar transfers for Windows OpenSSH hosts (#345)

[kipavy]

Closes #345.

What

Multi-item SFTP transfers to/from Win32-OpenSSH hosts use tar acceleration again, through the host's own tar.exe (bsdtar, shipped since Windows 10 1803), instead of the per-file fallback from #341.

How

• sftp_tar_available now detects the remote shell once per SFTP session (cached on the session entry):
  1. POSIX probe (unchanged): command -v tar && test -d /tmp
  2. echo __TF_TEMP__:%TEMP% — only cmd.exe expands it
  3. '__TF_TEMP__:' + $env:TEMP — only PowerShell expands it
  4. tar --version in the detected dialect
     All probes are harmless on any shell. The temp value must look like X:\..., otherwise it is rejected.
• New RemoteShell (commands/sftp/remote_shell.rs) writes tar commands for the detected shell:
  • SFTP /C:/a/b → native C:\a\b. A drive root becomes C:\. because a bare C: means the current directory on that drive.
  • Archives are staged under %TEMP% (in SFTP form for the upload/download step)
  • Exit codes: cmd branches && … || …, because %errorlevel% expands at parse time. PowerShell uses $LASTEXITCODE, captured before cleanup.
  • Symlinks are dereferenced when the extracting end is Windows (local tar for uploads, and the source host for remote→remote)
  • A command that goes over cmd.exe's 8191-character limit fails with a clear message and is not truncated
• Context-menu compress/extract use the same dialect, so they now work on Windows hosts too.
• POSIX command strings are byte-identical to before (the existing tests assert them exactly).

Testing

• cargo test --lib commands::sftp: 22 pass, with new tests for detection parsing, path mapping, quoting and per-shell status/cleanup. cargo clippy --lib is clean.
• I ran the PowerShell command forms for real in mcr.microsoft.com/powershell (pwsh 7): probe, tar --version with and without tar, create with ' and spaces in names, extract with mkdir and cleanup, and a failed extract reporting exit 2.
• Not tested against a real Windows host. I couldn't run cmd.exe or Windows PowerShell 5.1 here. This needs a manual check on a Win32-OpenSSH box, with both the cmd and PowerShell DefaultShell, before release.

Known limits

• In cmd.exe, a name containing %VAR% for a defined variable gets expanded inside the quotes (cmd has no escape for this).
• If a batch selection from a cmd.exe host goes over 8191 chars, it errors out; it does not fall back to per-file.