Pin all GitHub Actions to full commit SHAs

Org policy now requires actions pinned to immutable SHAs instead of
mutable version tags. Pin actions/checkout, actions/github-script,
softprops/action-gh-release, and oven-sh/setup-bun across all workflows.

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
       - name: Validate self-hosted runner guards
         run: ./tests/test_ci_self_hosted_guard.sh
@@ -23,10 +23,10 @@ jobs:
         working-directory: web
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
       - name: Setup Bun
-        uses: oven-sh/setup-bun@v2
+        uses: oven-sh/setup-bun@3d267786b128fe76c2f16a390aa2448b815359f3 # v2
 
       - name: Install dependencies
         run: bun install --frozen-lockfile
@@ -43,7 +43,7 @@ jobs:
       cancel-in-progress: false
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
         with:
           submodules: recursive