ci: skip breaking-changes entirely on push, not just its own check

[o-nnerb]

Summary

• The `breaking-changes` job already skips its own check on a push to `main` (the run step's own `github.event_name != 'pull_request'` guard) and already skips entirely on a PR labeled `breaking-changes` (the job-level `if:`) — but on push it still spins up a runner, checks out the repo, and restores the SwiftPM cache before noticing it has nothing to do and exiting 0. That reads as the check actually running (green, ~24s) when it's really a no-op, and wastes CI minutes on every push to `main`.
• Moves the `pull_request`-only guard into the job-level `if:`, next to the existing label-based skip, so a push to `main` skips the whole job (shown as skipped, not run) instead of spinning up and immediately exiting.
• Removes the now-unreachable runtime early-exit from the run step, since the job itself never executes on a non-PR event anymore.

Test plan

• `python3 -c "import yaml; yaml.safe_load(...)"` — valid YAML
• `actionlint` — clean
• Verify on the next push to `main` in a consuming repo that "💔 Breaking Changes" shows as skipped, not a fast green run