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/new-benchmark.md

@@ -76,7 +76,7 @@ class MyBenchmarkEnv(EnvAdapter):
         Args:
             item: The data item to process
             skill: Current skill document content
-            model: The student model instance
+            model: The target model instance
             
         Returns:
             TaskResult with prediction, score, and trajectory