Check tidy entirely on PR & diff on push

This addresses the issue where missing includes are not caught until
being merged. For a single push, we check the diff to ensure it doesn't
introduce serious tidy issues. The diff check is relatively lightweight.
For a PR, we perform a thorough check on the entire program to ensure
everything is fine before merging into the codebase.

.github/workflows/ci.yml

@@ -38,12 +38,12 @@ jobs:
           sudo apt-get update
           sudo apt-get install -q -y clang-tidy-18
       - name: static checks on diff
-        if: ${{ github.event_name == 'pull_request' }}
+        if: ${{ github.event_name == 'push' }}
         run: |
           export CLANG_TIDY_DIFF=clang-tidy-diff-18.py
           scripts/check-tidy-diff.sh
       - name: static checks
-        if : ${{ github.event_name == 'push' }}
+        if : ${{ github.event_name == 'pull_request' }}
         run: |
           export CLANG_TIDY=clang-tidy-18
           make tidy