feat: implement guarded Stellar Testnet deployment

[dDevAhmed]

Summary

Implements CLI-033 with a guarded stellarforge deploy MVP for Stellar Testnet smart-contract deployment.

Included

• registers stellarforge deploy
• requires explicit --network testnet
• requires a named Stellar CLI identity alias through --source
• supports generated Cargo smart-contract workspaces
• invokes Stellar CLI as an executable + argument array
• streams deployment output through the shared subprocess supervisor
• preserves failed Stellar CLI execution as a non-zero subprocess failure
• documents the Testnet-only deployment contract and security requirements
• tests exact deployment arguments, unsupported networks, unsafe source values, unsupported project layouts, output streaming, and failed deployment semantics

Security

• Mainnet, Futurenet, local networks, and implicit network defaults are rejected in the MVP
• raw secret keys, seed phrases, public-key-like values, paths, and malformed source values are rejected before spawning
• no shell interpolation
• no signing material is rendered or accepted as a raw StellarForge argument
• signing remains delegated to a preconfigured Stellar CLI identity

Closes #36