docs: create SDK_GUIDE.md with all 9 required sections

[lushan888]

What this adds

Creates docs/SDK_GUIDE.md — the missing SDK guide referenced by sdk/src/registry.ts when the find() method is called without an indexer integration.

Sections included

1. Installation — npm/yarn commands with peer dependency note
2. Quick Start (5 minutes) — minimal working example
3. Registering an Agent — complete example with all fields
4. Creating a Spending Policy — policy creation with limits
5. Executing an Autonomous Payment — agent-initiated payment
6. Checking Remaining Allowance — human-readable remaining budget
7. Error Handling — StellarMindError + ErrorCodes usage
8. Network Configuration — testnet/mainnet/custom RPC
9. TypeScript Types Reference — full type table matching actual exports

Verification

• All import statements match the actual exports in sdk/src/index.ts
• All type references match sdk/src/types.ts
• USDC_ISSUER references use the correct exported constant
• No placeholder text ("TODO", "coming soon") in the guide
• Every code snippet is syntactically correct TypeScript

Resolves #6