feat(contracts): implement token balance checks

[h1065153539-create]

Summary

/claim #11

Implements ContractClient.getMembershipTokenBalance so SDK users can query membership token balances through the configured JSON-RPC provider instead of receiving NOT_IMPLEMENTED.

Changes

• Validates wallet and contract addresses before making RPC calls.
• Requires rpcUrl and either a configured or per-call contractAddress with clear INVALID_CONFIG errors.
• Encodes balanceOf(address) calldata and sends a read-only eth_call.
• Decodes the returned uint256 balance and returns it as a decimal string.
• Surfaces RPC errors and malformed return data through existing SDK error codes.
• Documents balance lookup usage in README and API reference.

Testing

• npm.cmd run typecheck - passed
• npm.cmd run test:run - passed, 99 tests
• npm.cmd run build - passed
• npm.cmd run test:smoke - passed ESM/CJS/type smoke checks
• npm.cmd run lint - exited 0 with existing repository warnings only
• git diff --check - passed

Security / compatibility notes

• Uses standard JSON-RPC eth_call only; it does not sign transactions or mutate chain state.
• No secrets, private keys, API keys, or wallet credentials are logged or added.
• Existing guild owner and role requirement stubs keep their current behavior.

[h1065153539-create]

Refreshed this PR at �f60ab80 on top of current main (�fa3297) after the chain-aware contract config landed.

What changed in the refresh:

• Kept the token balance implementation, but now resolves
  pcUrl / contractAddress through getChainConfig(params.chainId).
• Preserved per-call contractAddress overrides and added optional per-call chainId support.
• Kept the newly merged chain config tests passing alongside balance-specific coverage.

Validation run locally:

pm run test:run -- tests/contracts.test.ts tests/client.test.ts -> 24 tests passed

pm run typecheck -> passed

pm run build -> passed

PR status after refresh: mergeable clean.

[Lakes41]

That's quite good, Please resolve conflicts