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

tsconfig.base.json

@@ -52,6 +52,7 @@
         "./packages/session-persistence/*/src",
         "./packages/ui/*/src",
         "./packages/util/*/src",
+        "./packages/mcp/*/src",
         "./packages/support/*/src"
       ]
     }