Reorganize CONTRIBUTING.md for better clarity

Move "Automatic Rejection" section to top of document as first-line
filter for reviewers. Consolidate duplicate rejection criteria from
"Common Mistakes" and bottom "Automatic Rejection" sections into
single comprehensive list. Reorder remaining sections in logical
flow: rejections, quality requirements, acceptance criteria, format
reference, review process.

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