feat: implement MCP stdio transport and tool registration

[qhkm]

Implement docs/plans/2026-02-27-mcp-stdio-transport.md:\n- add McpTransport trait + HttpTransport/StdioTransport\n- refactor McpClient to transport abstraction with stdio constructor\n- include stdio entries in discovery\n- extend McpServerConfig for stdio fields\n- wire MCP discovery/registration in create_agent()\n- update docs (CLAUDE.md + AGENTS.md) and run quality gates

[qhkm]

Implemented via docs/plans/2026-02-27-mcp-stdio-transport.md in local workspace changes (MCP transport abstraction, stdio transport, discovery + create_agent wiring, docs updates, and full test/clippy pass).