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.

README.md

@@ -1,6 +1,6 @@
 Quickly mount remote systems via SSHFS based on your ssh_config
 
-Static mount dir is currently `~/Servers/<Host>`
+Static mount dir is currently `~/Servers/<Host>` (uses the ssh_config alias as label, not the resolved IP).
 
 # Install
 
@@ -36,6 +36,7 @@ sshfsc <Host>
 | ---- | ----------- |
 | `-e` | open mountpoint in your editor |
 | `-v` | verbose: print resolved ssh_config fields (HostName, User, Port, IdentityFile) |
+| `-r`, `--remote-dir <path>` | remote directory to mount (default: remote home) |
 
 By default only the resolved mount path is printed. Use `-v` for the full
 ssh_config dump.