Add SDK quickstart guide

[xxCodexIAxx]

Summary

Adds docs/SDK_GUIDE.md, the SDK quickstart guide requested in #6.

Coverage

• Installation with npm/yarn
• Five-minute registry quickstart
• Agent registration with real SDK exports
• Spending policy creation
• Autonomous payment execution
• Remaining allowance display
• Error handling with StellarMindError and ErrorCodes
• Network configuration
• Exported TypeScript types reference

Validation

• Cross-checked imports against sdk/src/index.ts.
• Cross-checked type names against sdk/src/types.ts.
• Used USDC_ISSUER.testnet, which is exported from sdk/src/stellar.ts through the SDK entrypoint.
• No placeholder sections or TODO text left in the guide.

Bounty

Fixes #6

Please send the bounty payout to my Base wallet: 0x776CbBb63D81c08B4090DB503eAE7FDb8fBd7c6f