Replaces PR #6 after it became dirty against current main.

What changed:

• rebased the agent safety protocol contribution onto current main
• resolved the CONTRIBUTING.md conflict by keeping the current tools list and adding the safety/tool README guidance

Verification:

• python3 -m py_compile scripts/pr_safety_lint.py tools/memory-health/memory_health.py tools/proposal-pile/proposal_pile.py tools/receipt-log/receipt_log.py tools/receipt-log/viewer/build.py
• python3 scripts/pr_safety_lint.py --base origin/main --head HEAD
• git diff --check origin/main..HEAD
• markdown conflict-marker scan for README.md, CONTRIBUTING.md, AGENT-SAFETY.md