docs: add CLAUDE.md with build commands and architecture guide

[jaballer]

Summary

Adds a CLAUDE.md at the repo root to give Claude Code (and any new contributor) a fast, accurate mental model of the project. Focuses on the non-obvious things that require reading across multiple files.

What it documents

• Commands — build/test/lint scripts, and how to run a single unit test through ts-node (no build) vs. the full integration suite.
• Build/test coupling gotchas:
  • Integration tests shell out to the compiled ./dist/index.js and hit the live Postmark API, so they require a prior npm run build and real tokens in test/config/testing_keys.json. CI runs unit tests only.
  • preview/ isn't compiled — it's copied into dist/ by the syncPreview step, so preview edits need a rebuild.
• Architecture:
  • Two-layer yargs commandDir routing (thin parent command via the cmd() helper → subcommand directory).
  • Token/config flow: .env('POSTMARK') mapping env vars into hidden yargs args, with validateToken() prompting as fallback; ServerClient vs AccountClient.
  • The template manifest system (one directory per template: meta.json + content.html/content.txt, layouts under _layouts/) and how pull/push/preview revolve around it.
  • Output conventions (log(), CommandResponse, fatalError).
• Style — Prettier no-semicolons/single-quote/trailing-comma rules and strict ES5/CommonJS TypeScript.

Notes

Documentation only — no source or behavior changes.