ci(prettier): ignore package-lock.json

package-lock.json is machine-generated by npm; Prettier reformatting it
conflicts with npm's own output and causes the Prettier auto-commit
workflow to try (and fail) to push lockfile churn to protected develop.
Ignoring it lets the workflow find nothing to change and pass.

.prettierignore

@@ -1,3 +1,7 @@
 # Prettier auto-commit runs with GITHUB_TOKEN (GitHub App).
 # GitHub blocks that token from creating/updating anything under .github/workflows.
 .github/workflows/**
+
+# Machine-generated by npm; its format is owned by npm, not Prettier.
+# Prettier reformatting it fights npm and makes the auto-commit run fail.
+package-lock.json