Count the sentinel only when it is modified

This fixes the gate step skipping a pull request that adds test/needs-o2-dev
rather than edits it.

- Opting into the source build means editing the existing sentinel, so the
  check now uses a Modified-only file list.
- The pull request introducing the sentinel adds it, and so tripped its own
  opt-out and passed without running any test.
- The relevance check still uses the full list, so deletions keep counting.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: sawenzel

.github/workflows/sim-tests.yml

@@ -65,7 +65,12 @@ jobs:
         run: |
           set -eu
           changed=$(git diff --name-only "$BASE_SHA" "$HEAD_SHA")
-          if grep -qx 'test/needs-o2-dev' <<< "$changed" ; then
+          # Opting in means *editing* the existing sentinel, so only count it
+          # when it is Modified. A pull request that Adds it — which is what the
+          # pull request introducing the sentinel does — would otherwise trip its
+          # own opt-out and pass without testing anything.
+          opted=$(git diff --name-only --diff-filter=M "$BASE_SHA" "$HEAD_SHA")
+          if grep -qx 'test/needs-o2-dev' <<< "$opted" ; then
             echo "::notice title=Skipped::this pull request touches test/needs-o2-dev, so it is tested by build/O2DPG/sim/o2dev against O2 dev instead"
             echo "skip=true" >> "$GITHUB_OUTPUT"
           elif ! grep -qE '^(DATA/|MC/|test/|RelVal/)' <<< "$changed" ; then