feat: add Full Stack App starter template

[dDevAhmed]

Summary

Implements CLI-028 by adding the controlled Full Stack App starter with explicit frontend, backend, and contracts boundaries.

Included

• bundled full-stack template source
• frontend/ application boundary with smoke test
• backend/ Node.js configuration, health payload, and local HTTP entry
• contracts/ Soroban workspace boundary with guidance rather than fake production contract code
• public Stellar Testnet .env.example with no credentials
• root dependency-free check and test scripts
• generated README documenting structure, setup, environment, and secret-handling expectations
• automated generation smoke test through the real stellarforge new --template full-stack path

Security

• no wallet seeds, private keys, signing credentials, or production tokens
• no runtime/dev dependencies or lifecycle install scripts
• no network access required by smoke validation
• template remains project-controlled and flows through CLI-026's staged generation engine

Closes #31