feat: sftp, scp, and named port forwards

Add netssh sftp and netssh scp. Both resolve the host like a connection
does (cache, shortcuts and rewrites, instance/host, default user and
port, SSH options and rules, --jump/--no-jump, --dry-run) and get shell
completion. The per-client flag tables handle the differences from ssh:
the port is -P, -l is a bandwidth limit (the default user goes in as
-o User=), and IPv6 addresses are bracketed in host:path. All remote
scp operands must name the same host.

Add named port forwards under ssh.forwards (global, per instance, per
rule; a more specific layer replaces a forward of the same name),
netssh tunnel HOST NAME|SPEC... (ssh -N with ExitOnForwardFailure) and
netssh --forward NAME. Specs such as 8080:localhost:80 work without
configuration. Forwards are editable in netssh configure and completed
in the shell; the TUI opens SFTP with ctrl+o and a tunnel with ctrl+t.

Host resolution moves out of runSSHWrapper into resolveTarget, shared
by ssh, sftp, scp, tunnel, and the TUI.

Options added by netssh are now inserted before the first operand or
"--": after "--", ssh read them as the destination, and scp stops
reading options at the first operand.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ssh/options_test.go

@@ -20,7 +20,7 @@ func TestExtractWrapperFlags(t *testing.T) {
 		{[]string{"--no-jump", "--", "web"}, WrapperFlags{NoJump: true}, []string{"--", "web"}},
 	} {
 		flags, rest, err := ExtractWrapperFlags(tc.args)
-		if err != nil || flags != tc.flags || !reflect.DeepEqual(rest, tc.rest) {
+		if err != nil || !reflect.DeepEqual(flags, tc.flags) || !reflect.DeepEqual(rest, tc.rest) {
 			t.Errorf("%v: got %+v %v %v", tc.args, flags, rest, err)
 		}
 	}