Make mr-iid optional: auto-select single MR or prompt interactively

[mglaman]

Context

PR #293 added the mr:diff, mr:files, mr:status, and mr:logs commands. All of them require mr-iid as a positional argument. In practice, most issues have only one open MR, making the argument redundant noise.

Proposed behaviour

• Zero MRs — exit with a clear error message.
• One open MR — use it automatically (no argument needed).
• Multiple open MRs — in interactive mode, present a choice prompt (same pattern as issue:checkout branch selection); in non-interactive mode, list the available IIDs and exit with an error asking the user to specify one.

Implementation notes

• MrCommandBase::initialize() already resolves the issue NID and has access to $this->client and a GitLabClient. It can call ListMergeRequestsAction to fetch open MRs when mr-iid is not supplied.
• mr-iid must stay InputArgument::OPTIONAL (Symfony disallows a required argument after an optional nid), so the runtime validation in initialize() is the right place to add the auto-selection logic.
• The interactive prompt should use ChoiceQuestion on $mr->iid — $mr->title, consistent with the issue:checkout pattern.

Follow-up to

PR #293