ci(format): stage scoped file list in auto-commit step

The format job's "Commit formatting fixes" step previously fed the raw
`git diff --name-only` output into `git add`. If a tracked-but-gitignored
path (e.g. a build artifact) appeared in the diff, `git add` refused the
ignored path and aborted the step, so the auto-fix commit never landed
on the PR branch. PRs could then merge with formatting violations still
present, leaving main red on the post-merge push-scope check.

Stage only the paths listed in `.pr-format-files.existing.txt` — the
same scoped set the formatter operates on — so unrelated diffs in the
working tree can't poison the commit.

Author: marthakelly

.github/workflows/static_quality.yml

@@ -145,7 +145,13 @@ jobs:
             echo "No formatting changes to commit"
             exit 0
           fi
-          git diff --name-only -z | xargs -0 git add
+          # Stage only the files the formatter was scoped to operate on.
+          # Piping `git diff --name-only` into `git add` is unsafe: if a
+          # tracked-but-gitignored path shows up in the diff, `git add`
+          # aborts the whole step and the auto-fix push never lands —
+          # leaving formatting violations on the PR branch and (post-
+          # merge) on main.
+          xargs -a .pr-format-files.existing.txt git add --
           git commit -m "style: auto-fix formatting"
           git push