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/envs/alfworld/rollout.py

@@ -15,7 +15,7 @@ import time
 import concurrent.futures
 import numpy as np
 
-from skillopt.model import chat_student
+from skillopt.model import chat_target
 
 # ── Constants ─────────────────────────────────────────────────────────────────
 
@@ -210,7 +210,7 @@ def run_alfworld_batch(
 
         def call_api(idx):
             try:
-                response, _ = chat_student(
+                response, _ = chat_target(
                     system="You are an expert agent operating in the ALFRED Embodied Environment.",
                     user=prompts[idx],
                     max_completion_tokens=max_completion_tokens,