This commit adds the _internal package structure for AstrBot's
standardized MCP & Skills support:
astrbot/_internal/mcp/:
- MCPClient for MCP server connections
- MCPTool wrapper for MCP tools
- MCP configuration management
astrbot/_internal/skills/:
- SkillManager for skill lifecycle
- Skill parser and loader
- SkillToToolConverter for tool-based skills
- Prompt builder for skills
astrbot/_internal/tools/:
- ToolSchema, FunctionTool, ToolSet base definitions
- FunctionToolManager for tool registry
- Builtin tools (cron, send_message, kb_query)
- Tool providers (internal, plugin, computer)
astrbot/api/:
- Public API for tools (ToolRegistry, tool decorator)
- Public API for MCP (get_mcp_servers, register_mcp_server)
- Public API for skills (get_skill_manager, skill_to_tool)