feat(13-01): implement video type detection and Pydantic models

- 创建 SemanticSegmentationEngine 类
- 实现 VideoSegment 和 SegmentationResult Pydantic 模型
  - VideoSegment: title(可选), content(必填), start_index, end_index
  - SegmentationResult: video_type, segments[], actual_segment_count
  - 添加标题长度验证器（5-15字）
- 实现 _infer_video_type() 方法
  - 从元数据推断视频类型（知识类/故事类/访谈类/新闻类/vlog）
  - 支持从 title, category, tags 推断
  - 默认类型为"知识类"
- 实现 _build_prompt() 方法
  - 包含视频类型信息
  - 包含完整的分段要求（数量控制、句边界保护、标题生成等）
  - 包含 JSON 输出格式规范
- 添加 extract_json_from_llm_response() 工具函数
- 添加 AIClientProtocol 协议定义
- 添加 deferred-items.md 记录超出范围的测试

所有计划 13-01 测试通过（10个测试）

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.planning/phases/13-llm/deferred-items.md

@@ -0,0 +1,41 @@
+# Deferred Items - Phase 13
+
+## Plan 13-01: 视频类型识别
+
+### Deferred Tests (Out of Scope)
+
+The following tests in `tests/test_semantic_engine.py` are outside the scope of Plan 13-01:
+
+#### TestChineseSentenceBoundary Class (Lines 157-244)
+These tests belong to Plan 13-05 (中文句边界检测):
+- `test_chinese_sentence_boundary_punctuations` - Tests Chinese punctuation awareness
+- `test_prompt_emphasizes_semantic_pause` - Tests semantic pause emphasis
+- `test_prompt_not_only_punctuation` - Tests that boundaries aren't just punctuation
+- `test_sentence_boundary_in_segmentation` - Tests sentence boundary in segmentation
+
+**Reason:** These tests require implementing Chinese sentence boundary detection logic, which is part of Plan 13-05.
+
+**Action:** Skip these tests in Plan 13-01. They will be addressed in Plan 13-05.
+
+---
+
+### Additional Tests Implemented
+
+The following tests were added beyond the original plan but are reasonable extensions:
+
+#### TestPydanticModels - Title Length Validation
+- `test_title_length_validation_fails` - Tests that titles > 15 chars fail validation
+- `test_title_exactly_15_chars_passes` - Tests that titles == 15 chars pass validation
+
+**Reason:** The plan states "简洁描述型（5-15字）" for titles, so length validation is appropriate.
+
+**Action:** Implemented in Plan 13-01 as part of Pydantic model validation.
+
+---
+
+## Notes
+
+- Total tests in test_semantic_engine.py: 14
+- Tests for Plan 13-01: 8 (VideoTypeDetection + VideoTypeInPrompt + PydanticModels)
+- Deferred tests (Plan 13-05): 4 (TestChineseSentenceBoundary)
+- Extra tests (beyond plan but implemented): 2 (title length validation)