perf(lint): one ast-grep scan feeds both the lint gate and the flaky-test ratchet (#33903)

lint:ast-grep and check-flaky-test-pattern-ratchet each parsed the whole tree.
ast-grep-gates.mjs scans once; the lint gate re-runs ast-grep only on files with
warning-or-error findings (ast-grep's own report and exit code), and the flaky
ratchet counts test-* matches from the shared scan.

Co-authored-by: OrcaWin <293788423+OrcaWin@users.noreply.github.com>

.github/workflows/pr.yml

@@ -287,7 +287,7 @@ jobs:
         if: needs.code_paths.outputs.static_analysis == 'true'
         run: |
           node_modules/.bin/ast-grep test --config config/sgconfig.yml --skip-snapshot-tests
-          node_modules/.bin/ast-grep scan --config config/sgconfig.yml --min-severity warning --format github src config tests mobile
+          node config/scripts/ast-grep-gates.mjs --format=github
 
       # Why the full tree: it takes ~2s, and the pre-commit hook is skippable.
       - name: Check formatting