feat(templates): add .cursor/rules/echo_rules.mdc to echo-start

[CharlesWong]

Summary

Adds .cursor/rules/echo_rules.mdc to all 11 echo-start templates so that new projects created with echo-start include Cursor AI coding rules tailored to each template's architecture.

Closes #636

What's included

A rules file was added to every template with rules specific to that template's stack and patterns:

Template	Stack	Key rules
next	Next.js App Router	Echo server wiring, client vs. server imports, env vars, security
authjs	Next.js + Auth.js	Echo + Auth.js integration, session-based auth gating
next-chat	Next.js + AI SDK streaming	Streaming chat, EchoChatProvider, message state
next-image	Next.js + image generation	Image gen flows, multi-provider, file handling
next-video-template	Next.js + video generation	Async job pattern, multi-provider (Sora/Veo), rate limiting
assistant-ui	Next.js + assistant-ui	Root echo.ts wiring, NEXT_PUBLIC_ECHO_APP_ID usage
nextjs-api-key-template	Next.js + Prisma + API keys	createEchoOpenAI, per-user API key storage, streaming chat
react	React + Vite	EchoProvider setup, useEcho, EchoTokens, Vite env vars
react-chat	React + Vite + AI SDK	EchoChatProvider, streaming hooks, message UX
react-image	React + Vite + image gen	Image generation UX, prompt handling, display conventions
echo-cli	Node.js CLI (Commander)	Command architecture, auth flows, wallet handling, Clack prompts

Improvements over existing approaches

• Template-specific rules: Each template gets rules derived from its actual source code (imports, file structure, SDK usage), not generic copy-paste.
• Organized sections: Rules are grouped by concern (server wiring, env vars, security, TypeScript, etc.) for easier scanning.
• More comprehensive: Rules cover error handling, security, TypeScript patterns, and UX conventions in depth.
• Covers all 11 templates: Full coverage across every echo-start template.
• Based on real code: Rules reference actual exported names, file paths, and patterns from each template's source.

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[CharlesWong]

Updated: Added cursor rules to all 11 template directories

Changes in this update:

• Extended coverage from 1 template to all 12 — added customized .cursor/rules/echo_rules.mdc to: assistant-ui, authjs, echo-cli, next-chat, next-image, next-video-template, next, nextjs-api-key-template, react-chat, react-image, react
• Each template's echo_rules.mdc has template-specific content covering its unique SDK setup, integration patterns, environment variables, and framework conventions
• Rebased onto latest upstream/master