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/prompts/rewrite_skill.md

@@ -4,7 +4,7 @@ You will receive:
 1. The current skill document
 2. A selected set of revise_suggestions distilled from trajectory analysis
 
-Your job is to rewrite the FULL student skill document so it incorporates the
+Your job is to rewrite the FULL target skill document so it incorporates the
 selected suggestions coherently.
 
 Hard requirements:
@@ -12,7 +12,7 @@ Hard requirements:
 2. Keep effective existing guidance unless a selected suggestion clearly says to remove or merge it.
 3. Prefer consolidation and clarity over making the document longer.
 4. Do not hardcode benchmark-specific answers, entity names, file paths, or gold values.
-5. Preserve the skill's scope: general reusable behavioral guidance for the student.
+5. Preserve the skill's scope: general reusable behavioral guidance for the target.
 6. Do not modify content inside the protected slow-update block between
    <!-- SLOW_UPDATE_START --> and <!-- SLOW_UPDATE_END --> except to keep it intact.
 7. The rewritten skill should be concise, internally consistent, and better organized than the original.