feat(tools): add Model Context Protocol (MCP) client adapter for external tools

[Kaap10]

Problem Statement

Terminal Agent is currently restricted to its 12 built-in tools. Developers cannot connect external tool ecosystems (such as database query runners, browser inspectors, GitHub API integrations, or custom debuggers) into the agent loop.

Proposed Architecture & Solution

1. MCP Client Adapter (src/terminal_agent/tools/mcp.py):
   • Implement an MCP client communicating via stdio JSON-RPC with configured MCP servers.
   • Discover available tools using the MCP tools/list protocol.
   • Wrap MCP tool definitions as BaseTool instances compatible with ToolRegistry.
2. Configuration Support (src/terminal_agent/config/schema.py):
   • Add mcp_servers configuration to TerminalAgentConfig:

     mcp_servers:
       postgres:
         command: "npx"
         args: ["-y", "@modelcontextprotocol/server-postgres", "postgresql://localhost/mydb"]
       github:
         command: "docker"
         args: ["run", "-i", "--rm", "mcp/github"]
         env:
           GITHUB_PERSONAL_ACCESS_TOKEN: "${GITHUB_TOKEN}"

3. Registry Integration (src/terminal_agent/tools/registry.py):
   • Initialize and register active MCP server tools during ToolRegistry setup.
   • Cleanly handle MCP server shutdown on session completion.

Files to Modify / Create

• src/terminal_agent/tools/mcp.py (New)
• src/terminal_agent/tools/registry.py
• src/terminal_agent/config/schema.py
• tests/unit/test_mcp.py (New)

Acceptance Criteria

• MCP stdio server tools are discovered and converted to LLM-compatible tool schemas.
• Tool execution dispatches JSON-RPC tools/call requests and returns structured ToolResult.
• Graceful fallback and error handling if an MCP server fails to start or crashes.
• Mock MCP server unit test verifying schema generation and tool invocation.