fix(cli): use local ssh client for vm access

[LopatinDmitr]

Description

Remove the embedded native SSH/SCP implementation from the virtualization CLI.

d8 v ssh and d8 v scp now always use the local OpenSSH/SCP clients through d8 v port-forward --stdio=true as ProxyCommand.

This change also:

• supports passing additional local SSH/SCP client options after --, for example d8 v ssh vm -- -o StrictHostKeyChecking=no -R 8080:127.0.0.1:8080;
• keeps explicit CLI mappings for common options: --identity-file/-i is passed to OpenSSH/SCP as -i, and --known-hosts is passed as UserKnownHostsFile;
• keeps --local-ssh as a deprecated no-op compatibility flag;
• keeps --local-ssh-opts as a deprecated compatibility flag;
• updates CLI docs, user guides, and unit tests.

Why do we need it, and what problem does it solve?

We do not want to support and maintain two separate SSH/SCP clients in the virtualization CLI.

The embedded native implementation has limited compatibility and may behave differently from the OpenSSH client users already have configured locally. Keeping only the local OpenSSH/SCP path makes VM access more predictable: it uses the user's existing SSH configuration, agent, certificates, authentication behavior, and client options.

OpenSSH is also available on all supported user platforms, including Windows, so the CLI no longer needs a separate built-in SSH/SCP implementation as a fallback.

What is the expected result?

1. Run d8 v ssh user@vm -i /path/to/key and verify that the command authenticates through local OpenSSH.
2. Run d8 v scp -i /path/to/key ./file user@vm:/tmp/file and verify that SCP uses the same local-client path.
3. Run d8 v ssh -n myvms ubuntu-24 -p 2222 -- -o StrictHostKeyChecking=no -R 8080:127.0.0.1:8080 and verify that options after -- are passed to the local OpenSSH client.
4. Run d8 v ssh -n myvms ubuntu-24 -p 2222 --command xclock -- -o StrictHostKeyChecking=no -X -n and verify that the remote command and local OpenSSH options are both applied.
5. Run commands with deprecated --local-ssh or --local-ssh-opts and verify that they still work and print deprecation warnings.
6. Run d8 v ssh user@vm --known-hosts /path/to/known_hosts and verify that OpenSSH uses it as UserKnownHostsFile.

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: core
type: fix
summary: Use local OpenSSH and SCP clients for virtual machine SSH and SCP access.
impact_level: low

[diafour]

As client is not needed for "local ssh", we can report the error and proceed with "default" as namespace.