Architecture page with the request lifecycle #68

Why

AGENTS.md describes the package layout for people (and agents) editing the code, but nothing explains how a request flows through apic: parse, resolve variables, apply auth, send, capture, assert, persist. Contributors need it, and the course's final lesson ("how apic works inside") wants a picture to point at.

What

• docs/architecture.md under Reference: one diagram of the lifecycle (SVG, checked in, drawn with a tool that keeps the source next to it, e.g. D2 or Mermaid via the mkdocs-mermaid2 plugin), a paragraph per stage naming the package, and the three contracts (the dialect, --json, exit codes) with where each is enforced.
• The package table from AGENTS.md moves here and AGENTS.md links to it (one source).
• A "how to add a directive / a step / a command" checklist section, lifted from AGENTS.md conventions.

Where

docs/architecture.md, docs/assets/lifecycle.svg (plus its source), mkdocs.yml, AGENTS.md, CONTRIBUTING.md.

Tests

mkdocs build --strict; the docs link check.