feat: support external template referral codes

[clawdvader]

/attempt #612

Summary

• add support for external templates to ship echo.config.json with a referralCode / referral_code
• when echo-start --template https://github.com/... scaffolds an external template, it writes the referral code into .env.local using the detected framework public env var prefix
• remove echo.config.json from the scaffolded app after reading it so template metadata does not leak into the user project
• update the React referral hook to accept the docs-standard referral_code query param and public env referral variables generated by echo-start

Verification

• pnpm --filter echo-start build
• pnpm --filter @merit-systems/echo-react-sdk build
• git diff --check

Closes #612

[vercel]

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