improve mcp implementations

[wilsonwang371]

This pull request introduces comprehensive documentation and configuration improvements for task-level MCP (Model Context Protocol) server subset selection, enhancing both governance and flexibility. The changes clarify the policy model, document environment variable handling, and add concrete examples and recommendations in both English and Chinese. Additionally, a new example configuration for GitLab MCP integration is provided.

Documentation: Task-level MCP Subset Selection

• Added two in-depth design documents explaining task-level MCP server subset selection, including goals, architecture, configuration keys, subset composition logic, phased rollout, and security/compliance considerations. These are provided in both English (docs/mcp-task-subset-design-en.md) and Chinese (docs/mcp-task-subset-design-zh.md). [1] [2]

Policy Model and Configuration Enhancements

• Expanded documentation to clarify that task-level MCP policy defaults are automatically applied to chat session parameters, while invocation-level overrides are not yet supported. This is reflected in both English and Chinese integration architecture docs. [1] [2]
• Documented new session parameter keys for task-level tool allowlist/denylist (mcp.task_tool_allowlist, mcp.task_tool_denylist), indicating these are injected by the host and are read-only for the WASM guest. [1] [2] [3]

Environment Variable Handling

• Added detailed documentation for how environment variable references are resolved in MCP server configs, including required and defaulted references, shorthand for passing through environment variables, and behavior when variables are missing. Example and error handling are provided. [1] [2]

Example Configuration

• Added a new example configuration file for integrating a GitLab MCP server (config/sms/mcp.d/gitlab.toml), demonstrating how to set up stdio transport, environment variables, and resource budgets.

These changes improve clarity, governance, and operational best practices for MCP server selection and integration at the task level.