Fix scratchpad tool: rename op→action, add examples, improve error messages ...

- Rename primary parameter from 'op' to 'action' for consistency with
  all other tools (terminal, filesystem, todo, etc.). Legacy 'op' still
  works as a fallback to avoid breaking old calls in compressed context.
- Add JSON examples directly in the tool description so the model sees
  the exact structure to produce.
- Improve all error messages to include the correct JSON example,
  making it obvious what the model did wrong.
- Add manuals/scratchpad.md for tool_manual support.
- Update and expand tests for new syntax and error cases.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>