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/copilot-instructions.snippet.md

@@ -0,0 +1,33 @@
+<!--
+Copy this block into your repo's .github/copilot-instructions.md so Copilot
+knows the SkillOpt research-engine tools exist. (Copilot reads
+copilot-instructions.md automatically as ambient guidance.)
+-->
+
+## SkillOpt (research skill-optimization engine)
+
+This repo exposes the core **SkillOpt** training/eval engine via an MCP server
+(`skillopt`). SkillOpt is validation-gated, text-space skill optimization: it
+reflects on rollouts, makes bounded edits to a skill, and keeps a change only
+if it improves a held-out validation set.
+
+When the user asks to "optimize a skill", "train on <benchmark>", "run
+SkillOpt", "evaluate this skill", or "what configs can I run", use the MCP
+tools:
+
+- `skillopt_list_configs` — list the benchmark YAML configs you can pass as `config`
+- `skillopt_train` — run a reflective skill-optimization loop on a config (long-running; spends API/compute budget)
+- `skillopt_eval` — evaluate a single skill markdown file on a dataset (no training)
+
+Guidance:
+- Always run `skillopt_list_configs` first if you don't already know a valid `config` path.
+- `skillopt_train` and `skillopt_eval` are long-running and consume the user's
+  model backend/budget — confirm the `config`, `backend`, and model choices
+  with the user before launching, and surface the held-out gate result when the
+  run finishes.
+- For one-off YAML overrides use `cfg_options` (e.g. `seed=123 batch_size=40`);
+  for any other underlying flag use `extra_args`.
+
+This is distinct from the **SkillOpt-Sleep** MCP server (`skillopt-sleep`,
+`sleep_*` tools), which evolves a local coding agent from past sessions rather
+than running the research benchmarks.