ci: prefer PR-driven triggers and deduplicate runs via concurrency group

[Copilot]

Force-pushes to PR branches were not reliably triggering CI because push triggers exclude feature branches, and pull_request triggers had no mechanism to cancel stale runs. The result was silent CI gaps on active PRs.

Changes

• pull_request before push in C.yml, python.yml, R.yml, matlab.yml — PRs are the primary CI signal for feature branches; ordering reflects intent.
• push retained for main and v[0-9]+* only — CI still runs on merge and release branches; python.yml retains its release: trigger for wheel publishing.
• Concurrency group updated across all four workflows:

# Before
group: ${{ github.workflow }}-${{ github.ref }}

# After — deduplicates by PR number; force-pushes cancel stale runs
group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}

docs-preview-cleanup.yml, rchk.yml, and revdepcheck.yml are unchanged — they use workflow_call/workflow_dispatch/closed-PR triggers that don't follow this pattern.