docs: add Echo Cursor rules to templates

[realkoreanbeef]

/claim #636

Summary

• add .cursor/rules/echo_rules.mdc to all 11 echo-start templates
• tailor the rule content for Next.js, React/Vite, chat, image, video, API-key/database, assistant-ui, CLI, and Auth.js templates
• include Echo-specific guidance for safe SDK boundaries, secrets handling, paid action UX, validation, and template quality checks

Verification

• corepack pnpm exec prettier --parser markdown --check "templates/**/.cursor/rules/*.mdc" ✅
• python3 validation confirmed all 11 expected template rule files exist and include the shared Echo guidance ✅
• corepack pnpm install --frozen-lockfile attempted, but the repository prepare script failed in this environment with Unable to find package manager binary: cannot find binary path after dependencies installed. No implementation code was changed; this PR only adds Cursor .mdc rules.

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