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/exec.go

@@ -9,11 +9,14 @@ import (
 
 // Exec replaces the current process with the native ssh client via syscall.Exec.
 // All existing SSH configs, keys, and agent forwarding remain intact.
-func Exec(args []string) error {
-	sshPath, err := exec.LookPath("ssh")
+func Exec(args []string) error { return SSH.Exec(args) }
+
+// Exec replaces the current process with the client's binary.
+func (c Client) Exec(args []string) error {
+	path, err := exec.LookPath(c.Name)
 	if err != nil {
-		return fmt.Errorf("ssh not found in PATH: %w", err)
+		return fmt.Errorf("%s not found in PATH: %w", c.Name, err)
 	}
-	argv := append([]string{"ssh"}, args...)
-	return syscall.Exec(sshPath, argv, os.Environ())
+	argv := append([]string{c.Name}, args...)
+	return syscall.Exec(path, argv, os.Environ())
 }