fix: pin GitHub Actions to SHA for supply chain security (#452)

## Summary
- Pin all GitHub Actions `uses:` references to commit SHAs for supply
chain security
- Original version tags preserved as inline comments for maintainability
- Mitigates supply chain attacks where a compromised tag could inject
malicious code (ref: Trivy incident March 2026)

## Changes
- All `uses: owner/action@tag` → `uses: owner/action@SHA # tag`
- No version changes, only pinning format

## Test plan
- [x] Verify CI workflows run successfully
- [x] Confirm no action versions changed, only pinning format

Author: apham0001

.github/workflows/label-issues.yml

@@ -10,7 +10,7 @@ jobs:
     permissions:
       issues: write
     steps:
-      - uses: actions/github-script@v8
+      - uses: actions/github-script@ed597411d8f924073f98dfc5c65a23a2325f34cd # v8
         with:
           script: |
             github.rest.issues.addLabels({

.github/workflows/test.yml

@@ -9,6 +9,6 @@ jobs:
     # Ensures default fresh checkout can create containers.
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v6
+    - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
     - run: docker compose up --no-start
     - run: docker compose -f docker-compose.yml -f compose-debug.yml up --no-start