Add TeamAPI and improve error handling for v0.2.0

[delano]

Summary

This release expands the SDK from a single-purpose email sender to a comprehensive Lettermint client with two distinct API surfaces:

• SendingAPI — Project-level email sending with fluent builder (existing functionality, Client alias preserved for backward compatibility)
• TeamAPI — Team-level management for domains, projects, routes, webhooks, messages, suppressions, and stats

Also improves error handling by introducing ResponseParsingError with original exception preservation, following the pattern established by ConnectionError and WebhookJsonDecodeError. This addresses review feedback about preserving debugging context when API responses fail to parse.

What's New

• Lettermint::TeamAPI client with resource accessors for all team management endpoints
• Lettermint::ResponseParsingError wraps Faraday::ParsingError with original_exception attribute
• Comprehensive documentation: docs/api.md (HTTP reference) and docs/sdk.md (Ruby SDK reference)
• Updated error hierarchy in CLAUDE.md reflecting all current exception types

Backward Compatibility

• Lettermint::Client remains as alias to SendingAPI
• Existing code catching Lettermint::Error will still catch the new ResponseParsingError
• No breaking changes to the email sending interface

Test Plan

• Run bundle exec rspec — all 447 specs pass
• Verify ResponseParsingError preserves original exception
• Confirm Client alias works for existing integrations

[gemini-code-assist]

Code Review

This pull request updates the Lettermint Ruby SDK to version 0.2.0, introducing a significant architectural change by splitting the client into SendingAPI and TeamAPI. It adds new resource infrastructure, raw HTTP methods for direct API access, and a specific ResponseParsingError for improved error handling. Additionally, extensive documentation for both the Team API and the SDK has been added. A review comment suggests simplifying the error message for the new ResponseParsingError to avoid redundancy with the class name.