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>

skillopt/optimizer/__init__.py

@@ -1,4 +1,4 @@
-"""ReflACT Optimizer -- skill update operations.
+"""SkillOpt Optimizer -- skill update operations.
 
 Analogous to the optimizer in neural network training: applies the computed
 "gradient" (patches) to the current skill document to produce an updated
@@ -8,8 +8,8 @@ Modules
 -------
 - skill: edit application (optimizer.step() / parameter update)
 - clip: edit ranking and selection (gradient clipping)
-- meta_reflect: epoch-level macro refinement (momentum)
 - slow_update: longitudinal comparison and guidance (EMA / regularization)
+- meta_skill: cross-epoch memory for optimizer context
 """
 from skillopt.optimizer.skill import apply_edit, apply_patch  # noqa: F401
 from skillopt.optimizer.clip import rank_and_select  # noqa: F401