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/rewrite.py

@@ -1,9 +1,9 @@
-"""Teacher-driven full skill rewrite from selected revise_suggestions."""
+"""Optimizer-driven full skill rewrite from selected revise_suggestions."""
 from __future__ import annotations
 
 import json
 
-from skillopt.model import chat_teacher
+from skillopt.model import chat_optimizer
 from skillopt.prompts import load_prompt
 from skillopt.optimizer.update_modes import get_payload_items
 from skillopt.utils import extract_json
@@ -40,7 +40,7 @@ def rewrite_skill_from_suggestions(
     )
 
     try:
-        response, _ = chat_teacher(
+        response, _ = chat_optimizer(
             system=actual_system,
             user=user,
             max_completion_tokens=max_completion_tokens,