chore(distribution): auto-sync PAD release date from CHANGELOG

Program_Release_Month/Day/Year in website/pad.xml aren't a simple
version-substring bump, so release-please's extra-files marker can't
set them directly. Instead, read the date release-please just wrote
into CHANGELOG.md on its own release PR branch and push a follow-up
commit there with the matching PAD date fields -- that branch isn't
protected the way main is, so no privileged token is needed, and the
date lands on main normally when the release PR is merged.

Refs #143