feat: add coverage reporting to CI

- Run npm run coverage in test workflow (non-blocking)
- Upload coverage artifacts for visibility
- Uses existing c8 configuration with 80% baseline

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/reusable-test.yml

@@ -149,6 +149,21 @@ jobs:
         env:
           CLAUDE_CODE_PACKAGE_MANAGER: ${{ inputs.package-manager }}
 
+      - name: Run coverage
+        run: npm run coverage
+        continue-on-error: true
+        env:
+          CLAUDE_CODE_PACKAGE_MANAGER: ${{ inputs.package-manager }}
+
+      - name: Upload coverage reports
+        if: always()
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: coverage-${{ inputs.os }}-node${{ inputs.node-version }}-${{ inputs.package-manager }}
+          path: |
+            coverage/
+            *.lcov
+
       - name: Upload test artifacts
         if: failure()
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1