feat(FN-3366): document CI shard contract in contributing guide

Documentation update to `docs/contributing.md` adding details on the CI shard contract (FN-3366 Step 3).

Fusion-Task-Id: FN-3366

.github/workflows/pr-checks.yml

@@ -9,10 +9,60 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
-  checks:
-    name: Lint, Typecheck, Test
+  lint:
+    name: Lint
     runs-on: ubuntu-latest
 
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install pnpm
+        uses: pnpm/action-setup@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v5
+        with:
+          node-version: "24"
+          cache: pnpm
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Lint
+        run: pnpm lint
+
+  typecheck:
+    name: Typecheck
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install pnpm
+        uses: pnpm/action-setup@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v5
+        with:
+          node-version: "24"
+          cache: pnpm
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Typecheck
+        run: pnpm typecheck
+
+  test-shards:
+    name: Test shard ${{ matrix.shard }}/3
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        shard: [1, 2, 3]
+
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -32,11 +82,5 @@ jobs:
       - name: Install Bun
         uses: oven-sh/setup-bun@v2
 
-      - name: Lint
-        run: pnpm lint
-
-      - name: Typecheck
-        run: pnpm typecheck
-
-      - name: Test (full suite)
-        run: pnpm test:full
+      - name: Test (deterministic shard)
+        run: pnpm test:ci:shard --shard ${{ matrix.shard }} --total 3