Add environment-specific .env.example variants

[james2177]

Description

There's a single .env.example covering all environments. As Soroban config (network, RPC URL, contract ids) and the signing-key config from issue #16 grow, a single generic example makes it easy to accidentally point a testnet-configured local setup at mainnet contract ids by mistake.

Requirements and context

• Add .env.testnet.example and .env.mainnet.example variants with correct, distinct defaults for each, and update the README's setup instructions to reference the right one

Suggested execution

1. Fork the repo and create a branch

   git checkout -b dx/env-example-variants
   

2. Implement changes
   Relevant files:

• .env.testnet.example
• .env.mainnet.example
• README.md

3. Test and commit

   • Run npm run lint && npm run typecheck && npm test && npm run test:e2e
   • Include test output / screenshots or logs demonstrating the fix in the PR

   Example commit message:

   feat: add environment-specific .env.example variants
   

Guidelines

• Assignment required before starting — comment to claim this issue
• PR description must include: Closes #[this issue's number]
• Timeframe: 96 hours from assignment
• Complexity: High (200 points)

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Part of the Drips Wave for vortex-backend.