ci: cancel superseded PR runs, skip heavy jobs on non-code PRs

[shtse8]

Cuts CI waste (Ops org sweep), repo-specific until the shared SylphxAI/.github template lands.

• Superseded runs are cancelled only on pull requests (cancel-in-progress: ${{ github.event_name == 'pull_request' }}). Main pushes, merge-queue runs and release workflows are never cancelled.
• No paths/paths-ignore on any workflow that produces a required check. Test contents are unchanged.
• Verified with actionlint and the repo's own check scripts. The new filter job runs for the first time on this PR.
• The three-OS test steps are skipped for non-code-only PRs (LICENSE, CHANGELOG, CONTRIBUTING, SECURITY, issue templates). The matrix job still runs, so the required test () checks keep reporting. The Pages deploy no longer cancels in progress.