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/gradient/__init__.py

@@ -1,4 +1,4 @@
-"""ReflACT Gradient -- trajectory analysis and patch generation.
+"""SkillOpt Gradient -- trajectory analysis and patch generation.
 
 Analogous to gradient computation in neural network training: analyzes
 minibatch rollout trajectories to produce skill-edit patches (the "gradient"
@@ -8,10 +8,8 @@ Modules
 -------
 - reflect: minibatch trajectory analysis (gradient computation)
 - aggregate: hierarchical patch merging (gradient aggregation)
-- deep_probe: diagnostic probe generation (gradient probing)
 """
 from skillopt.gradient.reflect import (  # noqa: F401
     run_minibatch_reflect,
 )
 from skillopt.gradient.aggregate import merge_patches  # noqa: F401
-from skillopt.gradient.deep_probe import generate_deep_probe_instruction  # noqa: F401