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/setup/wizard_test.go

@@ -542,3 +542,29 @@ func TestSaveShortcutRewritesRoundtrip(t *testing.T) {
 		t.Fatalf("got %+v", loaded.Shortcuts.Rewrites)
 	}
 }
+
+func TestSaveForwardsRoundtrip(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	t.Setenv("HOME", t.TempDir())
+	global := map[string]config.Forward{"web": {Local: "8080:localhost:80"}, "socks": {Dynamic: "1080"}}
+	cfg := config.Config{
+		NetBoxes: []config.NetBoxConfig{{Name: "prod", URL: "https://prod.example", Token: "t", SSH: config.SSHOptions{Forwards: map[string]config.Forward{"db": {Local: "5432:localhost:5432"}}}}},
+		Resolver: config.ResolverConfig{Strategies: []string{"primary_ip"}},
+		SSH: config.SSHConfig{
+			SSHOptions: config.SSHOptions{Forwards: global},
+			Rules:      []config.SSHRule{{Match: config.SSHMatch{Tag: "dmz"}, SSHOptions: config.SSHOptions{Forwards: map[string]config.Forward{"back": {Remote: "9000:localhost:9000"}}}}},
+		},
+	}
+	if err := save(cfg); err != nil {
+		t.Fatal(err)
+	}
+	loaded, err := config.Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(loaded.SSH.Forwards, global) ||
+		!reflect.DeepEqual(loaded.NetBoxes[0].SSH.Forwards, cfg.NetBoxes[0].SSH.Forwards) ||
+		!reflect.DeepEqual(loaded.SSH.Rules[0].Forwards, cfg.SSH.Rules[0].Forwards) {
+		t.Fatalf("forwards lost: %+v / %+v / %+v", loaded.SSH.Forwards, loaded.NetBoxes[0].SSH.Forwards, loaded.SSH.Rules)
+	}
+}