docs: overhaul README and add project documentation assets

- Rewrite README.md and README_CN.md with comprehensive project documentation
  including core features, architecture overview, benchmarks, tech stack,
  quick start guide, and detailed installation instructions
- Add CONTRIBUTING.md with contribution guidelines
- Add architecture.svg and directory-structure.svg diagrams
- Add generated PNG assets for hero banner, core features, pain points,
  architecture, and benchmark results
- Add screenshot PNGs for installation guide (PostgreSQL, Neo4j, Alembic,
  API docs, frontend UI)
- Replace external GitHub image URLs with local asset references

CONTRIBUTING.md

@@ -0,0 +1,73 @@
+# Contributing to MemoryBear
+
+感谢你对 MemoryBear 的关注！我们欢迎任何形式的贡献。
+
+## 如何贡献
+
+### 报告问题
+
+- 使用 [GitHub Issues](https://github.com/SuanmoSuanyangTechnology/MemoryBear/issues) 提交 Bug 报告或功能建议
+- 提交前请先搜索是否已有相同的 Issue
+
+### 提交代码
+
+1. Fork 本仓库
+2. 创建功能分支：`git checkout -b feature/your-feature-name`
+3. 提交更改：遵循 [Conventional Commits](https://www.conventionalcommits.org/) 格式
+4. 推送分支：`git push origin feature/your-feature-name`
+5. 创建 Pull Request
+
+### Commit 格式
+
+```
+<type>(<scope>): <description>
+
+[optional body]
+```
+
+**Type 类型：**
+
+| Type | 说明 |
+|------|------|
+| `feat` | 新功能 |
+| `fix` | Bug 修复 |
+| `docs` | 文档更新 |
+| `style` | 代码格式（不影响逻辑） |
+| `refactor` | 重构（非新功能、非修复） |
+| `perf` | 性能优化 |
+| `test` | 测试相关 |
+| `chore` | 构建/工具链变更 |
+
+**示例：**
+
+```
+feat(extraction): add ALIAS_OF relationship for entity deduplication
+fix(search): correct hybrid search ranking when activation values are missing
+docs(readme): update architecture diagram with generated images
+```
+
+### 开发环境
+
+```bash
+# 后端
+cd api
+pip install uv && uv sync
+source .venv/bin/activate
+pytest  # 运行测试
+
+# 前端
+cd web
+npm install
+npm run lint  # 代码检查
+npm run dev   # 开发服务器
+```
+
+### 代码规范
+
+- Python：遵循 PEP 8，行宽不超过 120 字符
+- TypeScript：通过 ESLint 检查
+- 提交前确保测试通过
+
+## 行为准则
+
+请保持友善和尊重。我们致力于为所有人提供一个开放、包容的社区环境。