Studio Agents Suite: Add starter agents.md file amd ship it to site root

[katinthehatsite]

Related issues

Fixes https://linear.app/a8c/issue/STU-1332/create-starter-agentsmd-template
Related to https://linear.app/a8c/issue/STU-1334/implement-auto-placement-of-agentsmd-in-site-root

Proposed Changes

This PR implements the AGENTS.md file to the root of Studio sites. The file includes guidance on Studio CLI commands and usage, WordPress development practices as well as SQLite database documentation.

Testing Instructions

• Pull the changes from this branch
• Start Studio with npm start
• Create a Studio site
• Navigate to the Overview tab and open the site in Finder
• Confirm that AGENTS.md file is present in the root of the site
• Next, open a Terminal and cd into the site
• Start, for example, Claude
• Ask Claude about the database integration
• Confirm that you get the answers related to SQLite database integration
• Ask Claude about Studio CLI commands
• Confirm that it gets answers based on the AGENTS.md file

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[katinthehatsite]

For now, we do not overwrite the file. We will consider some more flexibility here to make sure the users get the most recent versions of AGENTS.md shall we have to modify it. It will be solved under https://linear.app/a8c/issue/STU-1332/create-starter-agentsmd-template