ScriptKit: add usage() smart unknown-command handler

- ScriptKit.usage(msg, scriptName): progressive fallback for unknown commands
  1. Keyboard-weighted fuzzy suggestions (QWERTY adjacency, 4-tier costs)
  2. Prefix matching (input is prefix of command = always suggest)
  3. Topic suggestions (prefix/substring only, no fuzzy)
  4. Fallback: Show All Commands button
- Updated changelog, README, TODO