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/merge_failure_rewrite.md

@@ -7,7 +7,7 @@ Merge guidelines:
 2. Resolve conflicts by keeping the more general, better-justified direction.
 3. Preserve unique high-impact corrective insights.
 4. Suggestions supported by many source patches should receive higher support_count.
-5. The output suggestions should help a later teacher rewrite the full skill.
+5. The output suggestions should help a later optimizer rewrite the full skill.
 
 Respond ONLY with a valid JSON object:
 {
@@ -17,7 +17,7 @@ Respond ONLY with a valid JSON object:
       "type": "add_rule|remove_rule|merge_rules|reorganize|compress|clarify",
       "title": "<short title>",
       "motivation": "<why this matters>",
-      "instruction": "<what the rewriting teacher should change in the skill>",
+      "instruction": "<what the rewriting optimizer should change in the skill>",
       "priority_hint": "high|medium|low",
       "support_count": <integer>,
       "source_type": "failure"