Don't guard in-place edit scripts with assert: PYTHONOPTIMIZE removes it

A common agent pattern is python3 - <<'EOF' ... assert old in s; write(s.replace(old, new)) EOF && git commit .... If PYTHONOPTIMIZE=1 is set in the environment (or the script runs under python3 -O), the assert is stripped. The script exits 0 after writing the file back unchanged, so the && chain commits anyway (tested on CPython 3.12.3). Use an explicit if old not in s: sys.exit('anchor not found'), which exits 1 in both modes. Also quote the heredoc delimiter (<<'EOF'): with <<EOF, bash expands $VAR and backticks inside the Python source, which corrupts markdown anchors.