feat: add Echo Cursor rules to templates

[clawdvader]

Summary

• Adds .cursor/rules/echo_rules.mdc to all 11 Echo templates
• Tailors guidance for Next.js, React/Vite, CLI, chat, image, video, Auth.js, and assistant-ui templates
• Covers Echo SDK usage, environment variable boundaries, API route/client-server separation, validation, testing, and editing checklists

Verification

• git diff --check passed
• corepack pnpm -C packages/sdk/echo-start run build passed

/claim #636

[vercel]

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