feat: add Stellar smart contract starter template

[dDevAhmed]

Summary

Implements CLI-029 by adding the trusted MVP Stellar smart-contract starter template and wiring it into bundled template resolution.

Included

• Cargo workspace rooted at the generated project
• starter contract under contracts/hello
• soroban-sdk workspace dependency
• simple non-custodial contract plus native unit test
• generated README using current stellar contract build, deploy, and invoke terminology
• Testnet-oriented examples with no embedded secret keys, seeds, or credentials
• generated-template smoke coverage using the real stellarforge new --template smart-contract path
• Cargo metadata validation without requiring network access

Security

• template is bundled and selected through the controlled registry only
• no arbitrary template paths or URLs
• no private keys, seed phrases, or deployment credentials
• starter contract intentionally contains no financial/custodial logic
• generation continues to use CLI-024/CLI-026 destination and file-copy protections

Closes #32