feat(protocols): implement MCP JSON-RPC transport layer

Add complete MCP protocol implementation:
- mcp_types.rs: JSON-RPC types, initialize, tools, resources, prompts
- mcp_transport.rs: Stdio-based transport with split mutexes for stdin/stdout
- McpServerConfig builders for npx/node/python MCP servers
- Full McpClient trait implementation for tools/resources/prompts
- Add McpError variant to ZclawError

Transport supports:
- Starting MCP server processes via Command
- JSON-RPC 2.0 request/response over stdio
- Length-prefixed message framing
- Tool listing and invocation
- Resource listing and reading
- Prompt listing and retrieval

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/zclaw-protocols/src/lib.rs

@@ -3,7 +3,11 @@
 //! Protocol support for MCP (Model Context Protocol) and A2A (Agent-to-Agent).
 
 mod mcp;
+mod mcp_types;
+mod mcp_transport;
 mod a2a;
 
 pub use mcp::*;
+pub use mcp_types::*;
+pub use mcp_transport::*;
 pub use a2a::*;