App custom blocks

Before creating, modifying, or troubleshooting custom block code, read node_modules/@notionhq/apps/skills/custom-blocks/SKILL.md. Resolve this path relative to the App project root. Follow the instructions in that file.

If the file is missing, install the project dependencies. If it remains missing, update @notionhq/apps to a release that includes the skill. Do not continue with custom block changes until you can read the SDK skill.