[codex] Remove PrimeCert bookkeeping

[Vilin97]

Summary

Finish the non-content cleanup after removing PrimeCert from Lean Pool content in #226.

This removes the stale NOTICE attribution and candidate-table/manual-list entries, and changes the candidate decision for b-mehta/primecert to include:false because the upstream project is actively maintained.

This also fixes the newly added project-advisory workflow so PRs with no added or changed LeanPool/projects.yml entries skip comment creation instead of failing while trying to post an empty advisory.

CI scope

This PR is non-content-only, so it is intentionally separate from #226 to satisfy content-pr-guard.yml.

Validation

• cd python && uv run --group test pytest tests/test_pr_advisory.py
• cd python && uv run ruff check lean_pool/pr_advisory.py tests/test_pr_advisory.py
• cd python && uv run ruff format --check lean_pool/pr_advisory.py tests/test_pr_advisory.py
• cd python && uv run python -m lean_pool.pr_advisory --pr-number 228 --dry-run
• Parsed candidates/decisions.jsonl locally
• rg -n "PrimeCert|primecert|Primecert" . now only finds the explicit include:false candidate decision

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