[feature] Replace --add flag with an `add` subcommand

[JerryAgbesi]

What problem are you trying to solve?
Adding a host entry today is done via the root command with the --add <alias> flag plus a positional user@host[:port] target (e.g. skipper --add bastion ubuntu@10.0.0.1:22). This conflates two surfaces on the same command: the default root behavior (open the TUI and connect) and a one-shot write operation. It is awkward to discover from --help, harder to grow (future options like -i/identity-file, comments, overwrite semantics), and inconsistent with the mental model users have from tools like git or gh where write operations live under their own verb.

Proposed solution
Introduce a dedicated add subcommand that replaces --add entirely:

skipper add <alias> <user@host[:port]> [flags]

• <alias> and <user@host[:port]> are required positionals.
• Reuses existing connect.ParseTarget + sshconfig.AddHost plumbing.
• Respects the existing -c, --config persistent flag for target config path.
• Prints the same "added host …" confirmation line as today.
• Remove the -a, --add root flag and its handling in cmd/root.go (runRoot's Changed("add") branch, the addAlias var, the addHost helper's tie-in).
• Update README flag table and any help text/examples.

Alternatives considered

• Keep --add and also add a subcommand (deprecation alias). Rejected: the project is pre-1.0 and adding both doubles the surface and docs.
• Fold into an interactive "add" flow from the TUI. Out of scope for this issue — that can be a follow-up on top of the subcommand.

Additional context

• Existing implementation: cmd/root.go (addHost, addAlias, flag registration in init), internal/sshconfig/writer.go (AddHost).
• Tests: add coverage for the new add subcommand (happy path + missing alias + malformed target) and drop tests tied to the removed flag.
• Breaking change for users of --add; call it out in the PR description and README.