Implement skill — test-integration-standards

Implements the `test-integration-standards` agent skill.

**Skill file:** `skills/test-integration-standards/SKILL.md`

Sections to implement:
- Test tier definition (unit / integration / e2e)
- When NOT to mock in integration tests — use real containerised infra, mock only external third-party services
- Real resources — approved patterns (Testcontainers, LocalStack, WireMock)
- Isolation and cleanup — per-test teardown, no execution order dependencies, container lifecycle
- CI behaviour — runnable without manual setup, unit/integration separation, slow test gating
- E2E specifics — critical journeys only, no shared state, reproducible failure artifacts