test(hooks): run session-start payload test in CI

- Problem: session-start-test.sh was not referenced by any workflow, so envelope regressions never failed CI.
- Fix: add a validate-skills step that runs the test and skips when jq is missing.
- Verification: bash hooks/session-start-test.sh → session-start JSON payload OK

.github/workflows/test-plugin-install.yml

@@ -40,6 +40,14 @@ jobs:
       - name: Test reference-link validator
         run: node --test scripts/validate-reference-links-test.js
 
+      - name: Test session-start hook payload
+        run: |
+          if ! command -v jq >/dev/null 2>&1; then
+            echo "jq not found; skipping session-start hook test"
+            exit 0
+          fi
+          bash hooks/session-start-test.sh
+
   validate-commands:
     name: Validate command parity and description sync
     runs-on: ubuntu-latest