feat: add MCP client plugin (dsh-mcp-client)

Connects to an external MCP server and registers its tools on
ctx.tools. Supports stdio (child process) and Streamable HTTP
transports. Credential-shaped env vars are scrubbed before forwarding
to child processes.

- Plugin lifecycle: connect, sync tools, re-sync on ToolListChanged,
  dispose unregisters and closes
- Full JSDoc on all exports (@param/@returns on functions)
- 100% per-file coverage (apply lifecycle, args coercion, env scrubbing)
- Config catalog regenerated

docs/module-graph.md

@@ -85,6 +85,9 @@ flowchart TD
   subgraph group_code_runtime["packages/code-runtime"]
     pkg_code_runtime["code-runtime"]
   end
+  subgraph group_mcp["packages/mcp"]
+    pkg_mcp_client["mcp-client"]
+  end
   pkg_llm --> pkg_brand
   pkg_bash --> pkg_brand
   pkg_llm_deepseek --> pkg_llm