ARCH6: Add docs/architecture.md (layers, DI, contribution guide)

[carloseberhardt]

Goal

Provide contributors with a clear mental model of the extension’s structure and coding conventions.

Acceptance Criteria

• New file docs/architecture.md covering:
  1. High‑level layers: Commands → Services → CLI / VS Code API → Utils.
  2. Dependency Injection pattern (service registry, how to mock).
  3. Error flow (StepZenError hierarchy + handler).
  4. Logging setup and log‑level setting.
  5. Testing strategy (Mocha + @vscode/test‑cli + mocks).
  6. Future roadmap hooks (telemetry, language server).
• README “Contributing” section links to the new doc.
• Doc renders correctly on GitHub (check tables, code fences).
• Commit: docs: add architecture guide & link from README.