gitignore: catch stray playwright artifacts at the repo root

`playwright/.gitignore` already excludes `test-results/` and
`playwright-report/` relative to `playwright/`. But playwright
artifacts can land at the repo root if the suite is ever invoked
from there (e.g., via a debug script that imports playwright
directly outside the test runner), leaving an untracked
`./test-results/.last-run.json` cluttering `git status`.

Adding the same entries at the repo-root .gitignore catches those
cases too — defensive and zero behavior change.

.gitignore

@@ -14,3 +14,9 @@ data
 
 # Web vault
 web-vault
+
+# Playwright artifacts (the suite is invoked from playwright/ so its
+# own .gitignore catches in-tree paths; this catches stray artifacts
+# that end up at the repo root when playwright is invoked elsewhere).
+test-results
+playwright-report