Add CI test stage with coverage reporting and test infrastructure

- Updated .github/workflows/ci.yml to include:
  - Go module caching for faster builds
  - Coverage report generation and upload to Codecov
  - 80% coverage threshold check
- Created tests/ directory with integration test framework
- Added test fixtures and configuration
- Initial integration test passes

Related: FRE-8b42289c (Pop: Add CI test stage to workflow)

tests/fixtures/test-config.yaml

@@ -0,0 +1,22 @@
+# Test configuration for Pop CLI integration tests
+
+app:
+  name: "Pop Test"
+  version: "1.0.0-test"
+
+api:
+  base_url: "http://localhost:8080"
+  timeout: 30s
+  retry_count: 3
+
+database:
+  driver: "sqlite"
+  path: ":memory:"
+
+mail:
+  provider: "test"
+  from_address: "test@frenocorp.com"
+
+logging:
+  level: "debug"
+  format: "json"