feat(knowledge): 完成 L0+L1 混合检索集成

核心功能：
- 新增 FrontmatterParser 解析 YAML frontmatter
- 新增 KnowledgeIndexService L0 内存索引
- 新增 LookupKnowledgeTool 混合检索工具
- 增强 DocumentManagementService 文件保存和索引同步

技术实现：
- 数据库迁移 V004: api_document.metadata (TEXT)
- 依赖新增: snakeyaml 2.0
- 配置新增: knowledge.base-path
- 可观测性: requestId 追踪 + 性能日志

质量保证：
- 单元测试: 31/31 通过
- 测试覆盖: FrontmatterParser(11), KnowledgeIndexService(13), LookupKnowledgeTool(7)
- 启动验证: L0 索引正常加载

归档文档：
- OpenSpec: openspec/changes/lookup-knowledge-integration/
- devflow 档案: devflow/projects/2026-06-24-lookup-knowledge-integration/
- handoff: handoff/2026-06-24-lookup-knowledge-integration.md

src/test/java/com/superbiz/agent/service/FrontmatterParserTest.java

@@ -0,0 +1,149 @@
+package com.superbiz.agent.service;
+
+import com.superbiz.agent.dto.Frontmatter;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * FrontmatterParser 单元测试
+ */
+class FrontmatterParserTest {
+
+    private FrontmatterParser parser;
+
+    @BeforeEach
+    void setUp() {
+        parser = new FrontmatterParser();
+    }
+
+    @Test
+    void testHasFrontmatter_withValidFrontmatter() {
+        String content = "---\ntitle: Test\n---\nContent";
+        assertTrue(parser.hasFrontmatter(content));
+    }
+
+    @Test
+    void testHasFrontmatter_withoutFrontmatter() {
+        String content = "# Just a title\nContent";
+        assertFalse(parser.hasFrontmatter(content));
+    }
+
+    @Test
+    void testHasFrontmatter_nullContent() {
+        assertFalse(parser.hasFrontmatter(null));
+    }
+
+    @Test
+    void testHasFrontmatter_emptyContent() {
+        assertFalse(parser.hasFrontmatter(""));
+    }
+
+    @Test
+    void testParse_validFrontmatter() {
+        String content = """
+            ---
+            title: 支付网关错误码
+            keywords: [ERR_TIMEOUT, 超时, 支付网关]
+            summary: 记录了支付网关所有核心错误码
+            category: api
+            ---
+
+            # 正文内容
+            """;
+
+        Frontmatter result = parser.parse(content);
+
+        assertNotNull(result);
+        assertEquals("支付网关错误码", result.getTitle());
+        assertEquals(3, result.getKeywords().size());
+        assertTrue(result.getKeywords().contains("ERR_TIMEOUT"));
+        assertEquals("记录了支付网关所有核心错误码", result.getSummary());
+        assertEquals("api", result.getCategory());
+    }
+
+    @Test
+    void testParse_withoutFrontmatter() {
+        String content = "# Just content\nNo frontmatter here";
+        assertNull(parser.parse(content));
+    }
+
+    @Test
+    void testParse_missingRequiredFields() {
+        String content = """
+            ---
+            title: Only Title
+            ---
+            Content
+            """;
+
+        // 缺少 keywords 和 summary，应返回 null
+        Frontmatter result = parser.parse(content);
+        assertNull(result);
+    }
+
+    @Test
+    void testParse_malformedYaml() {
+        String content = """
+            ---
+            title: Test
+            keywords: [unclosed array
+            ---
+            Content
+            """;
+
+        // YAML 格式错误，应返回 null
+        Frontmatter result = parser.parse(content);
+        assertNull(result);
+    }
+
+    @Test
+    void testParse_noClosingDelimiter() {
+        String content = """
+            ---
+            title: Test
+            keywords: [test]
+            summary: Test summary
+
+            Content without closing ---
+            """;
+
+        // 缺少结束标记，应返回 null
+        Frontmatter result = parser.parse(content);
+        assertNull(result);
+    }
+
+    @Test
+    void testParse_windowsLineEndings() {
+        String content = "---\r\ntitle: Test\r\nkeywords: [test]\r\nsummary: Summary\r\n---\r\nContent";
+
+        Frontmatter result = parser.parse(content);
+
+        assertNotNull(result);
+        assertEquals("Test", result.getTitle());
+    }
+
+    @Test
+    void testParse_withOptionalFields() {
+        String content = """
+            ---
+            title: Test Document
+            keywords: [test, doc]
+            summary: A test document
+            version: 1.0.0
+            author: Test Author
+            ---
+            Content
+            """;
+
+        Frontmatter result = parser.parse(content);
+
+        assertNotNull(result);
+        assertEquals("Test Document", result.getTitle());
+        assertEquals("1.0.0", result.getVersion());
+        assertEquals("Test Author", result.getAuthor());
+    }
+}