v0.4.3

Initial commit

This commit introduces the initial version of the Go SDK for the
Agent Client Protocol (ACP). It includes the core library for
building ACP agents and clients, generated type-safe wrappers from the
ACP schema, and connection handling logic.

A Nix-based development environment is provided via flake.nix for
consistent builds and formatting. The project is configured with
GitHub Actions for CI, including formatting checks, tests, and flake
validation.

Example agent and client implementations are included to demonstrate
usage and for integration testing. Documentation for contributing,
releasing, and using the SDK is also part of this initial commit.