CI: Move binary jobs off pull requests

Problem

Binary-related CircleCI jobs run on every pull request today, via the job list in pull-request.yml: build-binary, publish-binary, the kitchensink/staging/recipes matrix (multiple browser variants), npm-module verification, and binary-system-tests. These are the most expensive jobs in the pipeline, and they run on every PR push regardless of whether the change touches anything binary-related.

Goal

• Binary jobs no longer run on pull requests, full stop. No opt-in, no per-branch override, no path-based auto-enable for PRs that touch binary/packaging code — that gap is accepted.
• Binary jobs run on linux-x64 on every push/commit to develop.
• When a binary job fails, the author of the PR whose merge produced that commit is alerted.