Drop duplicate compatibility scan, gate scope before commenting

The OpenAPI descriptions published here are generated artifacts. A
required status check already runs a purpose-built breaking-change
differ against the authoritative source, across every release and on
dereferenced files, and blocks publication when it fires. Re-checking
the published output here duplicated that gate with a narrower, weaker
implementation that would drift from it over time, so remove it.

Removing the scan also removes the only third-party dependency, so drop
both pip-install steps and restore requirements.txt.

Move the out-of-scope file gate into the pin step so it runs before any
comment is posted. A candidate that touches files outside the
description directories no longer collects a comment announcing a merge
that will not happen, and the merge step no longer recomputes the same
diff. The commit is pinned and revalidated before merge, so evaluating
the gate earlier inspects exactly the content that gets merged.

Replace the scanner-derived comment with a plain inventory of the
changed files, which is what the comment was actually useful for.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 6b022cc6-cf92-4a1a-a283-cc4bcd70288c

Author: shawnHartsell