docs: add /mcp slash command to README command table

Author: RoyLin

README.md

@@ -250,6 +250,7 @@ Interactive session commands dispatched before the LLM. Custom commands via the
 | `/clear` | Clear conversation history |
 | `/history` | Show conversation turn count and token stats |
 | `/tools` | List registered tools |
+| `/mcp` | List connected MCP servers and their tools |
 
 ```rust
 use a3s_code_core::commands::{SlashCommand, CommandContext, CommandOutput};