chore(ui-tooling): drop the changeset polarity census's test and fixture corpus

First half of retiring ui/scripts/changeset-polarity-census.mjs, a report-only
census whose card closed once the instrument was rebuilt; no gate or workflow
runs it. Its test, the fixture corpus under
scripts/__tests__/fixtures/changeset-polarity/ and the markdown-test-inputs
row for that test go here; the script and its alias follow in the next PR
(the two halves together exceed the 3,000-line human-merge threshold).

Co-authored-by: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012eXtKYmXEnCc1GExP4go12

Author: claude