feat(sftp): migrate from paramiko to asyncssh (PPK v3, non-blocking IO)

[Orinks]

Summary

• Replace paramiko with asyncssh for all SFTP connections — asyncssh is non-blocking and handles PPK v2+v3 natively
• Remove ppk_utils.py and puttykeys dependency — no conversion layer needed, asyncssh reads PPK files directly
• Dedicated event loop on daemon thread eliminates UI freeze workarounds (_sftp_lock, _listdir_attr_safe, daemon threads) that paramiko required
• Simplify host_key_policy.py and ssh_utils.py — remove paramiko-specific code
• Update all SFTP tests to use asyncssh mocks (121 tests pass)

What changed

File	Change
pyproject.toml	Remove puttykeys, add asyncssh>=2.14
protocols.py	Rewrite SFTPClient with asyncssh + background event loop
ppk_utils.py	Deleted (asyncssh handles PPK natively)
host_key_policy.py	Simplified for asyncssh known_hosts param
ssh_utils.py	Remove paramiko-specific agent detection code
portkeydrop.spec	Update hidden imports for asyncssh
test_*.py (6 files)	Replace paramiko mocks with asyncssh mocks

Architecture

wx UI thread ──call──▶ SFTPClient.list_dir() ──_run()──▶ asyncio event loop (daemon thread)
                              │                                    │
                              │ blocks via Future.result()         │ await sftp.readdir()
                              ◀────────────────────────────────────┘

All asyncssh operations run on a persistent background event loop. The _run() helper submits coroutines and blocks until completion, keeping the wx UI thread responsive.

Preserved behavior

• Connect/disconnect, list_directory, upload, download, delete, rename, mkdir, rmdir, stat
• Key file auth (OpenSSH + PPK v2/v3), password auth, SSH agent auth
• Host key policies (AUTO_ADD, STRICT, PROMPT)
• FTP/FTPS clients untouched

Test plan

• 121 tests pass (all SFTP-related test files)
• Ruff check + format clean
• No remaining paramiko/puttykeys references in source
• Manual test: connect to SFTP server with OpenSSH key
• Manual test: connect to SFTP server with PPK v3 key
• Manual test: verify UI remains responsive during directory listing

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