feat: add --remote-dir flag and use ssh alias as mount label

- New `-r` / `--remote-dir` flag to mount a specific remote subdirectory;
  empty default preserves prior home-dir behaviour.
- Validate the flag value via a dedicated `rxRemoteDir` allowlist before it
  reaches the sshfs argv.
- Use the ssh_config alias (not the resolved HostName) as the local
  mountpoint name and as the sshfs source. File managers now show the
  human-readable label instead of the raw IP.
- Validate `args[0]` against `rxHostUser` since it now flows into argv.
- Rename `verify_mount_dir` parameter `hostname -> name` and `mount_sshfs`
  first parameter `hostname -> alias` for clarity.

main.go

@@ -20,13 +20,20 @@ import (
 var (
 	rxHostUser     = regexp.MustCompile(`^[A-Za-z0-9_][A-Za-z0-9._-]*$`)
 	rxIdentityFile = regexp.MustCompile(`^[A-Za-z0-9/~][A-Za-z0-9._@/:+=~%-]*$`)
+	rxRemoteDir    = regexp.MustCompile(`^[A-Za-z0-9/~][A-Za-z0-9._/~-]*$`)
 )
 
 var (
 	eFlag = flag.Bool("e", false, "open mountpoint in your editor")
 	vFlag = flag.Bool("v", false, "verbose: print resolved ssh_config fields")
+	rDir  string
 )
 
+func init() {
+	flag.StringVar(&rDir, "r", "", "remote directory to mount (default: remote home)")
+	flag.StringVar(&rDir, "remote-dir", "", "remote directory to mount (default: remote home)")
+}
+
 func main() {
 	flag.Usage = func() {
 		fmt.Fprintf(os.Stderr,
@@ -43,6 +50,12 @@ func main() {
 		os.Exit(2)
 	}
 
+	alias := args[0]
+	if err := validate_ssh_field("alias", alias, rxHostUser); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(3)
+	}
+
 	hostname, err := lookup_ssh_field(args[0], "HostName")
 	if err != nil {
 		fmt.Fprintln(os.Stderr, err)
@@ -96,8 +109,14 @@ func main() {
 		fmt.Fprintf(os.Stderr, "ssh config Port %q is not a valid port number\n", port)
 		os.Exit(3)
 	}
+	if rDir != "" {
+		if err := validate_ssh_field("remote-dir", rDir, rxRemoteDir); err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(3)
+		}
+	}
 
-	mount, err := verify_mount_dir(hostname)
+	mount, err := verify_mount_dir(alias)
 	if err != nil {
 		fmt.Fprintln(os.Stderr, "verify_mount_dir() failed:", err)
 		os.Exit(4)
@@ -109,6 +128,9 @@ func main() {
 		fmt.Println("Port:		", port)
 		fmt.Println("Ifile:		", ifile)
 		fmt.Println("Mount:		", mount)
+		if rDir != "" {
+			fmt.Println("Remote:		", rDir)
+		}
 		fmt.Println("---")
 	} else {
 		fmt.Println("Mount:	", mount)
@@ -120,7 +142,7 @@ func main() {
 		os.Exit(5)
 	}
 	if !chkmount {
-		if err := mount_sshfs(hostname, user, ifile, port, mount); err != nil {
+		if err := mount_sshfs(alias, user, ifile, port, mount, rDir); err != nil {
 			fmt.Fprintln(os.Stderr, "mount_sshfs() failed:", err)
 			os.Exit(6)
 		}
@@ -160,7 +182,7 @@ func run_editor(mount string) error {
 	return cmd.Start()
 }
 
-func verify_mount_dir(hostname string) (string, error) {
+func verify_mount_dir(name string) (string, error) {
 	homedir, err := os.UserHomeDir()
 	if err != nil {
 		return "", fmt.Errorf("resolve home dir: %w", err)
@@ -173,9 +195,9 @@ func verify_mount_dir(hostname string) (string, error) {
 	if err != nil {
 		return "", fmt.Errorf("resolve base %q: %w", base, err)
 	}
-	mount := filepath.Clean(filepath.Join(realBase, hostname))
+	mount := filepath.Clean(filepath.Join(realBase, name))
 	if !strings.HasPrefix(mount, realBase+string(os.PathSeparator)) {
-		return "", fmt.Errorf("hostname %q escapes mount base %q", hostname, realBase)
+		return "", fmt.Errorf("name %q escapes mount base %q", name, realBase)
 	}
 	if info, err := os.Lstat(mount); err == nil && info.Mode()&os.ModeSymlink != 0 {
 		return "", fmt.Errorf("mount path %q is a symlink", mount)
@@ -186,7 +208,7 @@ func verify_mount_dir(hostname string) (string, error) {
 	return mount, nil
 }
 
-func mount_sshfs(hostname string, user string, ifile string, port string, mount string) error {
+func mount_sshfs(alias string, user string, ifile string, port string, mount string, remoteDir string) error {
 	cmd := exec.Command("sshfs", "-p", port,
 		"-o", "IdentityFile="+ifile,
 		"-o", "idmap=user",
@@ -201,7 +223,7 @@ func mount_sshfs(hostname string, user string, ifile string, port string, mount
 		"-o", "StrictHostKeyChecking=accept-new",
 		"-o", "ServerAliveInterval=15",
 		"-o", "ServerAliveCountMax=3",
-		user+"@"+hostname+":", mount)
+		user+"@"+alias+":"+remoteDir, mount)
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
 	return cmd.Run()