fix(ssh): make --json actually emit JSON for remote scans

[Mapika]

Found while testing #63. portview ssh <host> --json has never worked, and it fails differently depending on the far end.

With portview installed remotely — fatal

--json is also the transport this side parses back, so it is injected into the remote command. The user's own flag was then appended to it:

$ portview ssh devbox --json
SSH error (devbox): error: the argument '--json' cannot be used multiple times

The remote clap rejects the repeat and the whole command dies. This is the common case — anyone scripting against a host that has portview installed.

Without portview remotely — silently ignored

Agentless collection succeeded and then rendered a table anyway, because run_ssh_scan unconditionally called display_port_table and never looked at the flag.

The fix

The injected flag and a user-typed one are indistinguishable once they are in the same list, so intent is captured first and the duplicate filtered out:

fn build_scan_args(remote_args: &[String]) -> (Vec<String>, bool) {
    let want_json = remote_args.iter().any(|a| a == "--json");
    let mut args = vec!["--json".to_string()];
    args.extend(remote_args.iter().filter(|a| *a != "--json").cloned());
    (args, want_json)
}

An empty result is now [] rather than No ports found on remote host. — this output gets piped, so prose on stdout is a parse error waiting to happen.

Verification

Both remote modes, against the same host, with the ss/portview availability on the far end varied deliberately:

== agentless --json (portview hidden on the far end) ==
[{"port":3000,"protocol":"TCP","pid":8,...,"local_addr":"127.0.0.1","start_time_unix":1785535092}]

== normal remote --json (portview present) ==
[{"port":3000,"protocol":"TCP","pid":8,...,"local_addr":"127.0.0.1","start_time_unix":1785535091}]

== no --json still renders a table ==
╭──────┬───────┬─────┬────────────── …

== empty result ==
[]

Byte-identical between the two paths apart from the timestamp. Three regression tests cover the arg construction, including the --json --json case that was fatal.

172 tests, fmt/clippy clean, macOS + Windows type-check.