Move slash command dispatch logic and 12 individual command handlers
(/export, /share, /copy, /name, /session, /changelog, /hotkeys,
/compact, /thinking, /edit-mode, /arminsayshi, plus showThinkingSelector)
into a new slash-command-handlers.ts module.
InteractiveMode now delegates to dispatchSlashCommand() via a
SlashCommandContext interface, keeping the integration surface minimal.
Handlers that are also invoked from keybindings/events remain on
InteractiveMode and are accessed through the context.
Reduces interactive-mode.ts from 4,783 to 4,272 lines (-511).
334a7cf076