Foundation ticket for the custom CLI slash-command system. Build a command registry/execution model for agent-local IdeA commands exposed to the custom CLI, starting with native commands such as /help and /clean. Product decision: /clean clears the current conversation/session view; do not introduce a separate /reset command unless it gains a clearly distinct product meaning. Must provide a stable contract to list commands, filter them by prefix, expose metadata needed by UI (name, short description, availability, whether confirmation is required), and execute a selected command. Goal: frontend should consume one unified command source, regardless of whether commands are native IdeA commands or future plugin-provided commands.