Add CLAUDE.md documenting codebase structure and workflows

[xmesutx]

Covers dev/test/lint commands, the content/data/translations model, request-rendering pipeline, and generated REST/GraphQL/webhook docs for future Claude Code sessions working in this repo.

Claude-Session: https://claude.ai/code/session_01DDqxsqz2YkydC6mnfJ5T2o

Why:

Closes:

What's being changed (if available, include any code snippets, screenshots, or gifs):

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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