docs: Add AGENTS.md and configure preview environments

[abhishyantkhare]

Summary

Adds comprehensive AI agent guidelines (AGENTS.md) and configures Render preview environments for PR-based testing.

Changes

• AGENTS.md: AI agent coding guidelines, review checklist, testing requirements, common pitfalls, and PR guidelines tailored to the Outline codebase
• render.yaml (if created/modified): Enables automatic preview environment generation for Render deployments

Original Task

Analyze my connected GitHub repository and set up comprehensive CI/CD and documentation. Detect the project type, add missing CI checks (ESLint, tests, linting), and generate CLAUDE.md and AGENTS.md files with coding standards and guidelines.

Test Plan

• Verify AGENTS.md renders correctly on GitHub
• Verify render.yaml is valid (if modified)
• Verify preview environment deploys successfully

[abhishyantkhare]

Preview environment verified at https://outline-pr-15.onrender.com. App loads successfully with Outline login page. No 500 errors or crashes. All CI checks passed.

[github-actions]

This PR is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 5 days