docs: add golden context files for AI agent navigation

[david-shibley-contentful]

Summary

• Adds AGENTS.md, ARCHITECTURE.md, and CONTRIBUTING.md to make this repo navigable for AI agents and new engineers without a human guide
• AGENTS.md covers ownership, structure, sharp edges, and never/always rules
• ARCHITECTURE.md documents the PostMessage SDK, UMD build pipeline, dual-publish setup (@contentful/app-sdk + contentful-ui-extensions-sdk), and extension lifecycle
• CONTRIBUTING.md documents the dev workflow, test conventions (Mocha + jsdom), and release process

Test plan

• Verify files render correctly on GitHub
• Spot-check that repo paths and commands referenced in the docs are accurate

Generated with Claude Code

[Jared Jolton (jjolton-contentful)]

Hey David nice work on the context docs! These will be super useful. Quick flag on the CI changes:

The registry switch from registry.npmjs.org → npm.pkg.github.com in the release job looks like it would stop publishing to public npm. This package has thousands of downstream consumers installing via npm install @contentful/app-sdk, if we stop publishing to the public registry, they'd silently stop receiving updates.

The unit job now pulls @contentful packages from GitHub Packages, meaning anyone cloning the repo without a GITHUB_PACKAGES_WRITE_TOKEN configured can't npm ci. The release job's .npmrc now points exclusively at GitHub Packages. The dual-publish script would push both package names there instead of public npm. The new npm-github registry config ties automated dep updates to GH Packages auth as well.

Curious if this maybe snuck in from a local environment config, if it is intentional, I think it deserves its own PR with a rollout plan, we'd want to make sure the dual-publish to public npm is preserved.

[david-shibley-contentful]

yeah that must have snuck in, that was not intentional. I will back out

[david-shibley-contentful]

Closing in favor of a clean branch — the original branch accidentally included two pre-existing commits from a GitHub Packages migration (not our changes). New PR with only the golden context docs: #2555 (or see the new branch chore/add-golden-context-v2)