Interactive Slash-Command Expansion Plugin (SuperPi.Extension behaviour).

Intercepts on_input/2 unless "disableSlashCommands" => true is set (--disable-slash-commands):

• /plan <task> — Rewrites prompt into structured step-by-step planning mode.
• /goal <task> — Rewrites prompt into autonomous thorough completion mode and enables goal verification.
• /boost <task> — Enables deep multi-perspective reasoning and architectural verification.
• /model <model_id> — Updates pi.agent model and handles command without calling LLM (:handled).
• /mode <auto|accept-edits|plan> — Updates pi.agent executionMode (:handled).