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-types/src/error.rs

@@ -46,6 +46,12 @@ pub enum ZclawError {
 
     #[error("Internal error: {0}")]
     Internal(String),
+
+    #[error("Export error: {0}")]
+    ExportError(String),
+
+    #[error("MCP error: {0}")]
+    McpError(String),
 }
 
 /// Result type alias for ZCLAW operations