Pure Dart core for AI Agent Playground. Agent execution engine, LLM adapters, MCP clients, and tool orchestration — no Flutter dependency. Changelog excerpt: - Added `McpAgentEngine.registerAgentFromManifest()`— builds and registers an `Agent`directly from a parsed `SkillManifest`, handling tool-declaration → `McpServerConfig`mapping and prompt-step conversion. Accepts optional `dartTools`, `workingDir`, and `serverOverrides`. - Updated `SkillExporter.toSkillMd()`compatibility tag from `mcp_playground`to `"Universal"`to match agentskills.io TealKit standard. - Added `dartTools`parameter to `registerAgentFromManifest()`for Dart-native local tool injec[...]