feat: add AGENTS.md to 5 templates (batch 1)

[zeke]

This PR adds AGENTS.md files to 5 templates picked as a representative sampling across different complexity levels and Cloudflare products. It's intended as a conversation-starter — if we like this approach, we can ship it and generate AGENTS.md files for all the other templates too.

Templates covered:

• d1-template (simple, D1)
• durable-chat-template (Durable Objects, WebSockets, PartyKit)
• react-router-starter-template (framework SSR, Vite)
• saas-admin-template (D1, Workflows, Astro, API auth)
• postgres-hyperdrive-template (Hyperdrive, external Postgres)

Each AGENTS.md covers:

• What the template does and when to use it
• Setup commands and prerequisites
• Configuration table of bindings, secrets, and environment variables
• How the architecture works and how to extend it
• Deployment steps
• Relevant official Cloudflare Skills and MCP servers (including the Cloudflare API MCP) specific to the products used in that template
• A note asking agents to keep the file up to date as the template changes

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✨ cc @kristianfreeman who pointed me at @CameronWhiteside's AGENTS.md on the pay-per-crawl template

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