refactor: rename teacher/student to optimizer/target, remove best skills, fix slow update

- Rename teacher -> optimizer, student -> target across all code, configs, docs, prompts
- CLI: --teacher_model -> --optimizer_model, --student_model -> --target_model
- Remove best_skill files, keep only initial skills
- Fix slow update gate (force write into skill)
- Fix SLOW_UPDATE marker stripping
- Remove deep_reflect and meta_reflect mechanisms
- Update .env.example with export prefix and azure_cli docs
- Add endpoint empty validation in azure_openai.py

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

docs/guide/skill-document.md

@@ -70,7 +70,7 @@ Track your skill's evolution through:
     1. **Start with a seed skill** (`env.skill_init`) if you have domain knowledge — it converges faster
     2. **Use cosine LR schedule** — aggressive early exploration + careful late refinement
     3. **Enable slow update** (`use_slow_update: true`) to prevent forgetting across epochs
-    4. **Enable meta skill** (`use_meta_skill: true`) so the teacher accumulates strategy memory
+    4. **Enable meta skill** (`use_meta_skill: true`) so the optimizer accumulates strategy memory
 
 ## Next Steps