New mcp_client.rs: McpRegistry manages MCP server connections.
Spawns child processes, speaks JSON-RPC 2.0 over stdio. Discovers
tools via tools/list, dispatches calls via tools/call.
dispatch_with_agent falls through to MCP after checking internal
tools. McpRegistry lives on Agent (shared across forks).
Still needs: config-driven server startup, system prompt integration.
Co-Authored-By: Proof of Concept <poc@bcachefs.org>
