feat(architecture): create CALM files — platform, list artifact DDD, toolchain

[MLKiiwy]

Summary

Create the initial CALM (Common Architecture Language Model) architecture files under architecture/ that describe the current Alistigo platform architecture. Split into reusable patterns and concrete system instantiations.

Files to create

• architecture/README.md — navigation, Mermaid overview diagrams, tooling instructions
• architecture/patterns/ddd-hexagonal.pattern.json — reusable DDD hexagonal layer pattern
• architecture/patterns/event-sourcing-cqrs.pattern.json — event sourcing + CQRS pattern
• architecture/systems/alistigo-platform.arch.json — four-tier platform view (actors, tiers, external services)
• architecture/systems/list-artifact-ddd.arch.json — list artifact internal DDD architecture (all packages as nodes + relationships)
• architecture/systems/monorepo-toolchain.arch.json — dev toolchain: Nx, Bun, Biome, TypeScript, Vitest, Playwright, CALM, CI

Acceptance criteria

• All files created and valid against CALM JSON Schema
• pnpm calm validate architecture/ passes
• Platform view covers all four tiers + external actors + external services
• List artifact view covers all packages with correct dependency relationships
• Toolchain view covers dev, QA, CI tools

Epic: architecture-as-code (Issue #116)