KAN-161: Add service contract template

[kmosoti]

Outcome

Create a reusable service contract template for core components.

Acceptance Criteria

• Template captures input, output, mutation, idempotency, failure modes, complexity, external cost, and provenance.
• Template is referenced from engineering standards.
• At least one upcoming service card shows how to fill it in.

Concept Focus

Contracts, black-box boundaries, and cost-aware design.

Source

docs/03-kanban-board.md KAN-161.