feat: implement project configuration schema

[dDevAhmed]

Summary

Implements CLI-034 by defining and enforcing the StellarForge MVP project configuration contract.

Included

• adds versioned stellarforge.config.json schema
• supports explicit network and named Stellar CLI identity
• resolves precedence as CLI override → project config → missing
• does not default missing network values
• loads configuration only from the project root
• rejects symlink/non-file configs and files above 64 KiB
• rejects unsupported fields and secret-bearing field names
• rejects raw Stellar G/S StrKey values as identities
• integrates deployment with project config while preserving Testnet-only deployment
• records the architecture in ADR-0005
• documents the public configuration contract
• tests valid, invalid, missing, precedence, redaction, StrKey, and symlink behavior

Security

• JSON is parsed, never executed
• no raw secret/signing material is accepted as project config
• errors do not echo config contents or secret values
• Mainnet is never selected implicitly
• command-specific network restrictions remain authoritative

Closes #37