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/skill.py

@@ -28,9 +28,19 @@ def _is_in_slow_update_region(skill: str, target: str) -> bool:
     return start_idx <= target_idx < region_end
 
 
+def _strip_slow_update_markers(text: str) -> str:
+    """Remove any SLOW_UPDATE markers from edit content to prevent duplication."""
+    return (
+        text.replace(SLOW_UPDATE_START, "")
+            .replace(SLOW_UPDATE_END, "")
+    )
+
+
 def _edit_fields(edit: EditType | dict) -> tuple[str, str, str]:
     op = edit.op if hasattr(edit, "op") else edit.get("op", "")
-    content = (edit.content if hasattr(edit, "content") else edit.get("content", "")).strip()
+    content = _strip_slow_update_markers(
+        (edit.content if hasattr(edit, "content") else edit.get("content", "")).strip()
+    )
     target = edit.target if hasattr(edit, "target") else edit.get("target", "")
     return op, content, target