CI: add PR hygiene checks using dannywillems/toolbox

[dannywillems]

Add pr-hygiene workflow with scripts from dannywillems/toolbox:

• PR size check (warn at 300 lines, fail at 500)
• Commit title length check (<= 80 chars)
• No fixup/squash/WIP commits check
• Commit body required for 20+ line changes

Replicates LeakIX/l9format-python#69.