docs: add Echo Cursor rules to templates

[wognsdl2]

Summary

• Adds .cursor/rules/echo_rules.mdc to every echo-start template.
• Tailors guidance for Next.js, chat, image/video generation, Auth.js, API-key, React/Vite, and CLI templates instead of sharing one generic rule file.
• Updates the templates README structure example to show the Cursor rules location.

Verification

• Confirmed all 11 template directories contain .cursor/rules/echo_rules.mdc.
• pnpm is not available in my local environment, so I could not run the repository format/build scripts. This change only adds Cursor rule markdown files plus one README line.

Closes #636

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