chore: clean repository history

.env.example

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+PORT=3000
+APP_DATA_DIR=.app
+COURSE_KB_ROOT=kb/python-course-kb-practical-python/wiki
+COURSE_KB_VERSION=kb-local
+ENABLED_BATCH=full
+SANDBOX_IMAGE=coding-mentor-python-runner:0.1.0
+SANDBOX_SERVICE_URL=
+SANDBOX_PORT=3001
+SANDBOX_TIMEOUT_MS=3000
+SANDBOX_PYTEST_TIMEOUT_MS=8000
+SANDBOX_MEMORY_MB=128
+SANDBOX_OUTPUT_BYTES=20000
+AI_PROVIDER=
+AI_BASE_URL=https://api.openai.com/v1
+AI_MODEL=gpt-5.5
+AI_API_KEY=
+AI_TIMEOUT_MS=30000
+AI_MAX_OUTPUT_TOKENS=1200
+AI_REASONING=