Pin GitHub Actions to full-length commit SHAs (#88)

Replace mutable major-version tags on external actions with verified
full-length commit SHAs from official upstream repositories to mitigate
supply-chain risk. Each pin stays within the same major version, so
workflow behavior is preserved.

- actions/checkout@v4        -> 34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
- biomejs/setup-biome@v2     -> 4c91541eaada48f67d7dbd7833600ce162b68f51 # v2.7.1 (annotated tag, peeled commit)
- actions/setup-node@v4      -> 49933ea5288caeca8642d1e84afbd3f7d6820020 # v4.4.0
- actions/upload-artifact@v4 -> ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: miguelcalderon

.github/workflows/biome.yml

@@ -13,11 +13,11 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
       - name: Check Biome Version
         run: ./scripts/check-biome-version.sh
       - name: Setup Biome
-        uses: biomejs/setup-biome@v2
+        uses: biomejs/setup-biome@4c91541eaada48f67d7dbd7833600ce162b68f51 # v2.7.1
         with:
           version: "1.9.4"
       - name: Run Biome

.github/workflows/playwright.yml

@@ -15,8 +15,8 @@ jobs:
     timeout-minutes: 60
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-node@v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+      - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4.4.0
         with:
           node-version: lts/*
       - name: Install dependencies
@@ -25,7 +25,7 @@ jobs:
         run: npm exec playwright install chromium --with-deps
       - name: Run Playwright tests
         run: npm run e2e-tests
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
         if: ${{ !cancelled() }}
         with:
           name: playwright-report