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

[LopatinDmitr]

Description

Drop the embedded Go SSH/SCP implementation and run the local ssh/scp
binaries directly through a ProxyCommand (the d8 port-forward subcommand).
Provide a uniform way to pass extra options to the local client and to run
a command on the VM.

Highlights of the user-facing surface:

• Single --ssh-args flag for the local ssh/scp client. Can be repeated
  or given as a single value with space-separated options:

  d8 ssh user@myvm --ssh-args='-o StrictHostKeyChecking=no -o LogLevel=ERROR'
  d8 ssh user@myvm --ssh-args='-o X' --ssh-args='-o Y'

• -- separator for the command to execute on the VM (mirrors plain ssh):

  d8 ssh user@myvm -- 'ls -la /'
  d8 ssh user@myvm -- bash -lc 'echo $HOSTNAME'

• -c/--command remains supported as an equivalent to --.
• --known-hosts is mapped to OpenSSH UserKnownHostsFile; --identity-file
  to -i.
• --local-ssh and --local-ssh-opts are kept as deprecated compatibility
  flags (--local-ssh-opts is the old name for --ssh-args).
• templates.MinimumArgs was added so commands can consume everything
  after --.

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

The previous virtctl-style command shipped its own native Go SSH/SCP
client, including terminal handling, knownhosts parsing and stdio
tunnel plumbing. Maintaining that implementation:

• duplicated what OpenSSH already does well (proxy commands, keep-alives,
  host key verification, compression, control master, etc.);
• made it impossible to use features the local client already supports
  (-L, -R, -D, jump hosts, custom config in ~/.ssh/config,
  ProxyJump, etc.) without growing the wrapper with more flags;
• meant every fix for an edge case (e.g. non-ASCII terminals on Windows)
  had to be ported to the embedded code.

Switching to the local ssh/scp client makes the CLI delegate to the
tooling users (and CI) already have, with a much smaller surface in Go
code. The new --ssh-args design replaces the previous --ssh-opts /
positional-tail approach, which was both verbose (each -o had to be
wrapped in its own flag) and inconsistent with the standard ssh CLI.

What is the expected result?

1. d8 ssh user@myvm opens an interactive SSH session through
   d8 v port-forward --stdio=true (ProxyCommand).
2. d8 ssh user@myvm -- 'cmd' runs cmd on the VM and exits with its
   status code; d8 ssh user@myvm -c 'cmd' does the same.
3. d8 ssh user@myvm --ssh-args='-o X -o Y' passes -o X -o Y to the
   local ssh (split into separate argv entries by whitespace).
4. d8 scp local user@myvm:remote copies a file via local scp; scp
   flags can be supplied via the same --ssh-args flag.
5. The deprecated --local-ssh-opts flag still works and emits a
   warning pointing to --ssh-args.

Manual smoke test (13 cases covering --ssh-args single/repeat/mixed
forms, -- vs -c, deprecation warning, scp up- and download) was run
against vm-alpine in the test namespace.

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: cli
type: fix
summary: "d8 ssh / d8 scp now use the local OpenSSH/SCP clients via ProxyCommand; pass extra client options through a single --ssh-args flag and the command via --."
impact_level: low