chore(qol): Restructure CONTRIBUTING.md and improve agent instructions:

- control what content gets processed by adding the
  appropriate tags in CONTRIBUTING.md--for example:\

  - <!-- agent:instruct: essential --> - Keep all content
  - <!-- agent:instruct: condense --> - Summarize to key points
  - <!-- agent:instruct: remove --> - Skip entirely

- rename and move script to helper-scripts

Author: jstirnaman