fix(sftp): handle Bitvise strict SFTP compliance for directory navigation

[Orinks]

Summary

Fixes #46 — cannot enter .ssh directory on Bitvise SSH Server after asyncssh migration.

Root cause: Bitvise negotiates SFTP v4+ where file type (directory, regular, symlink) is communicated via a separate attrs.type field rather than being embedded in Unix permission bits (attrs.permissions). The existing code only checked stat.S_ISDIR(attrs.permissions), which returns False when the type bits are absent — causing directories like .ssh to appear as regular files and making navigation silently fail (click tries download instead of chdir).

Changes:

• list_dir: check attrs.type (SFTP v4+ file type constant) as fallback when permissions lack file type bits, for both directory and symlink detection
• chdir: validate the target is actually a directory via stat() after realpath(), instead of blindly trusting the canonicalised path. Surfaces NotADirectoryError and PermissionError to the UI
• stat: use attrs.type fallback for is_dir detection

Test plan

• New TestSFTPBitviseCompliance class with 8 tests covering:
  • list_dir detects directories via type field when permissions are None
  • list_dir detects directories via type field when permissions lack S_IFDIR bits
  • Regular files with type=REGULAR stay as files (no false positives)
  • Symlinks resolved via type field fallback
  • chdir accepts directories identified via type field
  • chdir accepts directories identified via permissions (backward compat)
  • chdir surfaces PermissionError from stat
  • stat() detects directories via type field
• chdir rejects non-directories with NotADirectoryError
• chdir validates via stat() call
• All 362 existing tests still pass
• Ruff lint + format clean

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