fix: add CI gate to prevent .received.txt snapshot files from being committed #4880

Description

CLAUDE.md warns never to commit .received.txt files, but there is no automated enforcement:

• No CI step to fail if any .received.txt files exist in committed code
• No pre-commit hook to prevent .received.txt commits
• Developers could accidentally commit received files instead of verified files

Suggested Fix

• Add a CI workflow step: find . -name "*.received.txt" -exec false \; (fail if any exist)
• Add a .gitignore entry or pre-commit hook for *.received.txt
• Document the exact git commands for snapshot acceptance in contributing docs