/search Command

Implement the /search command. It should use the results that ytsr pulls and, in a yet to be determined way, allow the user to choose a song from those results to play or add to the queue.

Start by locating where slash commands are registered and dispatched, then trace how song playback and queueing is currently invoked. Add a `/search` command path in that same flow using ytsr result data, and wire a selection step that sends the chosen track into the existing play/add-to-queue behavior. The issue names no files or tests, so discovery is part of the work; completion is achieved when `/search` returns results and a user choice triggers playback or queueing.

Written by the indexing model from the issue text.