Add SkillOpt research-engine MCP server plugin for Copilot

Exposes scripts/train.py and scripts/eval_only.py as Copilot MCP tools
(skillopt_list_configs, skillopt_train, skillopt_eval) via a stdlib-only
stdio server, mirroring the existing SkillOpt-Sleep plugin layout.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

plugins/copilot/skillopt/mcp-config.example.json

@@ -0,0 +1,11 @@
+{
+  "mcpServers": {
+    "skillopt": {
+      "command": "python3",
+      "args": ["plugins/copilot/skillopt/mcp_server.py"],
+      "env": {
+        "SKILLOPT_REPO": "${workspaceFolder}"
+      }
+    }
+  }
+}