ci: split test job into 5 parallel vitest shards (#1618)

.github/workflows/ci.yml

@@ -30,6 +30,10 @@ jobs:
 
   test:
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        shard: [1, 2, 3, 4, 5]
 
     steps:
       - uses: actions/checkout@v4
@@ -42,7 +46,7 @@ jobs:
           cache: pnpm
 
       - run: pnpm install --frozen-lockfile
-      - run: pnpm run test
+      - run: pnpm run test --shard=${{ matrix.shard }}/5
 
   # pi-tui's suite runs on node:test (not vitest), so the root `pnpm run test`
   # does not execute it; it needs its own job.