Add tech design document templates, sample, and AI Agent instructions

Co-authored-by: bearguo <20483603+bearguo@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

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+# GitHub Copilot Instructions for Tech Design Documents
+
+## Overview
+
+This repository is used to create technical design documents using AI Agent mode in an IDE. When a user asks you to create a technical design document, follow the guidelines below.
+
+## Creating a New Technical Design Document
+
+### Step 1: Understand the Requirement
+
+Ask clarifying questions if needed to understand:
+- What feature or system is being designed?
+- What problem does it solve?
+- Who is the target audience for the document?
+- Any specific constraints (tech stack, timeline, team size)?
+
+### Step 2: Use the Template
+
+Always base new technical design documents on the template located at:
+```
+templates/tech-design-template.md
+```
+
+### Step 3: Save the Document
+
+Save the new document to the `docs/` directory using the naming convention:
+```
+docs/<feature-name>-tech-design.md
+```
+
+For example:
+- `docs/user-auth-tech-design.md`
+- `docs/payment-service-tech-design.md`
+- `docs/data-pipeline-tech-design.md`
+
+### Step 4: Fill in the Content
+
+When generating the document, ensure that:
+1. **All sections from the template are included** — do not skip sections; mark them as "N/A" if not applicable
+2. **The overview is concise** — 2-3 sentences maximum
+3. **Architecture diagrams** use ASCII art or reference image files stored in `docs/images/`
+4. **API designs** include request/response examples with realistic data
+5. **Security considerations** are always addressed, even for internal tools
+6. **Open questions** capture any unresolved decisions that need input from stakeholders
+
+### Step 5: Update Status
+
+Set the document status to `Draft` initially. The user will update it to `In Review` or `Approved` as appropriate.
+
+## Document Quality Standards
+
+- Use clear, concise language
+- Avoid jargon without explanation
+- Use tables for structured comparisons
+- Include diagrams for complex architectures (ASCII art is acceptable)
+- Reference external standards or RFCs where applicable
+- Ensure all links and cross-references are valid
+
+## Updating an Existing Document
+
+When a user asks to update an existing tech design document:
+1. Read the current document carefully
+2. Make targeted updates to the relevant sections
+3. Increment the version number (e.g., 1.0 → 1.1 for minor changes, 1.0 → 2.0 for major redesigns)
+4. Add a note in the document about what changed if the status is `In Review` or `Approved`
+
+## Example Prompts
+
+Users may interact with you using prompts like:
+
+- "Create a tech design doc for a new notification service"
+- "Write a technical design for migrating our database from MySQL to PostgreSQL"
+- "Generate a design document for the search feature refactor"
+- "Update the payment service tech design to add support for cryptocurrency"

README.md

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 # tech-design
-Tech design docs
+
+[English](#english) | [中文](#中文)
+
+---
+
+## 中文
+
+这个仓库用于在IDE里使用AI Agent模式创建技术设计文档。
+
+### 快速开始
+
+1. 在支持AI Agent模式的IDE中打开此仓库（例如：VS Code + GitHub Copilot）
+2. 进入 Agent 模式（在 GitHub Copilot Chat 中选择 `Agent` 模式）
+3. 向 Agent 描述你的技术设计需求，例如：
+   > "请根据 `templates/tech-design-template.md` 模板，为 [你的功能] 创建一个技术设计文档，保存到 `docs/` 目录。"
+4. Agent 会自动生成技术设计文档并保存到 `docs/` 目录下
+
+### 目录结构
+
+```
+tech-design/
+├── .github/
+│   └── copilot-instructions.md   # AI Agent 指令配置
+├── templates/
+│   └── tech-design-template.md   # 技术设计文档模板
+├── docs/
+│   └── sample-tech-design.md     # 示例技术设计文档
+└── README.md
+```
+
+### 模板使用说明
+
+- **模板文件**：`templates/tech-design-template.md`
+- **文档存放**：所有生成的技术设计文档保存在 `docs/` 目录
+- **命名规范**：`docs/<功能名称>-tech-design.md`，例如 `docs/user-auth-tech-design.md`
+
+---
+
+## English
+
+This repository is used to create technical design documents using AI Agent mode in an IDE.
+
+### Quick Start
+
+1. Open this repository in an IDE that supports AI Agent mode (e.g., VS Code + GitHub Copilot)
+2. Switch to Agent mode (select `Agent` mode in GitHub Copilot Chat)
+3. Describe your technical design requirements to the Agent, for example:
+   > "Please create a technical design document for [your feature] based on the `templates/tech-design-template.md` template, and save it to the `docs/` directory."
+4. The Agent will automatically generate the technical design document and save it to the `docs/` directory
+
+### Directory Structure
+
+```
+tech-design/
+├── .github/
+│   └── copilot-instructions.md   # AI Agent instructions configuration
+├── templates/
+│   └── tech-design-template.md   # Technical design document template
+├── docs/
+│   └── sample-tech-design.md     # Sample technical design document
+└── README.md
+```
+
+### Template Usage
+
+- **Template file**: `templates/tech-design-template.md`
+- **Document storage**: All generated technical design documents are saved in the `docs/` directory
+- **Naming convention**: `docs/<feature-name>-tech-design.md`, e.g., `docs/user-auth-tech-design.md`