Add a script checking changelog PR references

Changelog entries often omit the current PR number or pin a predicted
number that belongs to another PR.  Add a script that finds the entries
a PR introduces and fails when one lacks the PR number in its trailing
references or in the links metadata of its fragment.

An entry is introduced unless it existed before the PR, at the merge
base or on a branch the PR merged in: an entry with the same first
paragraph, or failing that, one editing an entry with the same first
line, existed before.  This excludes edits to historical entries that
keep their first line, as well as entries carried forward through a
merge, while still catching new entries.

#1236

Changelog: none
Assisted-by: Claude Code:claude-opus-5-5

Author: co2plant