refactor-dispatch

Single front door for improving existing code — routes to the right engine by mode: deslop (strip AI slop, code + product), code (safe behavior-preserving refactor), debt (tech-debt register), perf (frontend + backend optimization), structure (read-only structural review), or stack (dependency + framework-pattern modernization). Backs the /refactor command. Use when asked to refactor, clean up, de-slop, pay down tech debt, optimize, or modernize, and the mode must be picked from an argument like "deslop", "debt", "perf", or "stack".