ci: remove the CircleCI configuration

GitHub Actions is green on this branch (4/4 jobs), so CircleCI has nothing left to
do. Removing it here rather than in a follow-up because the reason to keep it
never materialised: the plan was to run both pipelines on the same commit and
compare, but CircleCI has been dormant since 2026-05-25. Its last run failed on
`assemble`, it posts no statuses, and 10 commits reached `dev` with no CI at all --
there was never a baseline to compare against.

Keeping a rewritten-but-unread config would imply CircleCI is a live fallback when
it is not: the project no longer follows this repo, so re-enabling it would be a
manual action either way.

This also drops the JUnit staging step, which existed only to build the directory
layout store_test_results required. upload-artifact takes the source path
directly, so it is deleted rather than ported.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: shamarisouthwell1