feat(templates): add comprehensive Cursor rules for Echo projects (closes #636)

[D3CC]

Description

Add comprehensive .cursor/rules/echo_rules.mdc to echo-start templates as requested in #636.

What's included:

SDK Initialization Patterns:

• Server-side Echo SDK setup in src/echo/index.ts
• Client-side EchoProvider wrapper patterns
• Environment variable requirements table

Server/Client Boundaries:

• 'use client' directive usage
• Next.js App Router API route patterns with Echo-billed providers
• Auth checking with isSignedIn() in server components

React Component Patterns:

• Functional components with TypeScript props
• Named exports convention
• Absolute import paths vs relative imports

TypeScript Configuration:

• Strict mode usage
• Interface vs type conventions
• Kebab-case filename convention

CLI Patterns:

• Echo TS SDK client initialization
• generateText() and streamText() usage

Additional Coverage:

• Testing patterns with Vitest
• Error handling for async operations
• Tailwind CSS styling conventions
• Dark mode theming

File

• templates/.cursor/rules/echo_rules.mdc — Cursor AI rules tailored for Echo Next.js, React, and CLI projects

References

• Modeled after the Convex example format shown in #636
• Based on actual Echo SDK patterns from templates/next-chat/src/echo/index.ts and templates/next-chat/src/providers.tsx

Closes #636

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