ci: add 70% coverage threshold gate

Phase 17 Batch 5: Fail CI if test coverage drops below 70%.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gcmsg

.github/workflows/ci.yaml

@@ -39,6 +39,16 @@ jobs:
       - name: Test
         run: CGO_ENABLED=1 go test -race -coverprofile=coverage.out ./...
 
+      - name: Check coverage
+        run: |
+          COVERAGE=$(go tool cover -func=coverage.out | grep total | awk '{print substr($3, 1, length($3)-1)}')
+          THRESHOLD=70
+          echo "Coverage: ${COVERAGE}% (threshold: ${THRESHOLD}%)"
+          if [ "$(echo "$COVERAGE < $THRESHOLD" | bc -l)" -eq 1 ]; then
+            echo "::error::Coverage ${COVERAGE}% is below threshold ${THRESHOLD}%"
+            exit 1
+          fi
+
       - name: Build
         run: CGO_ENABLED=1 go build -v ./cmd/peerclawd