[czpython]

Closes #85.

asyncssh's SFTP server answers limits@openssh.com from its own constants, a 4 MiB write, and never asks the SFTPServer. A client that honours the answer, such as asyncssh 2.24 with put at its default block size, sends a file under 4 MiB as one write packet. The sandbox's OpenSSH sftp-server ends the session on a packet of 256 KiB or more, so every upload of that size failed with SFTPConnectionLost.

Change

• The backend asks the sandbox server its limits after the handshake, in the order asyncssh's own client uses. A failure there tears the process down like a failed handshake.
• GatewaySFTPServer.limits returns that answer, and a caller's limits@openssh.com request is routed to it beside the existing extension filter. A sandbox server without the extension leaves the client's 16 KiB default in place.
• Two tests: the advertised write length fits the sandbox server's packet, and a 300 KiB put at the client's default block size round-trips. Both fail on main, the second with the production error.
• The test sftp-server also resolves at the Debian path, so the SFTP tests run on Linux instead of skipping. They skipped in CI until now.
• One sentence in the deploy guide, and the checklist's truthiness rule applied in the touched files.

Verification

uv run pytest: 857 passed. ruff check, ruff format --check, pyright: clean.