docs: add one-line update guide (#71)

- New docs/update.md for AI agents to follow
- README: add update one-liner next to install one-liner
- Same pattern: copy one line to your agent, it handles the rest

Co-authored-by: Panniantong <panniantong@users.noreply.github.com>

README.md

@@ -42,6 +42,12 @@ AI Agent 已经能帮你写代码、改文档、管项目——但你让它去
 
 复制给你的 Agent，几分钟后它就能读推特、搜 Reddit、看 YouTube、刷小红书了。
 
+**已经装过了？更新也是一句话：**
+
+```
+帮我更新 Agent Reach：https://raw.githubusercontent.com/Panniantong/agent-reach/main/docs/update.md
+```
+
 > ⭐ **Star 这个项目**，我们会持续追踪各平台的变化、接入新的渠道。你不用自己盯——平台封了我们修，有新渠道我们加。
 
 ### ✅ 在你用之前，你可能想知道
@@ -93,6 +99,11 @@ AI Agent 已经能帮你写代码、改文档、管项目——但你让它去
 
 就这一步。Agent 会自己完成剩下的所有事情。
 
+> 🔄 **已安装过？** 更新也是一句话：
+> ```
+> 帮我更新 Agent Reach：https://raw.githubusercontent.com/Panniantong/agent-reach/main/docs/update.md
+> ```
+
 > 🛡️ **担心安全？** 可以用安全模式——不会自动装系统包，只告诉你需要什么：
 > ```
 > 帮我安装 Agent Reach（安全模式）：https://raw.githubusercontent.com/Panniantong/agent-reach/main/docs/install.md

docs/update.md

@@ -0,0 +1,65 @@
+# Agent Reach — Update Guide
+
+## For Humans
+
+Copy this to your AI Agent:
+
+```
+帮我更新 Agent Reach：https://raw.githubusercontent.com/Panniantong/agent-reach/main/docs/update.md
+```
+
+---
+
+## For AI Agents
+
+### Goal
+
+Update Agent Reach to the latest version, then verify everything still works.
+
+### Step 1: Check current version
+
+```bash
+agent-reach check-update
+```
+
+If it says "已是最新版本", you're done — tell the user.
+
+### Step 2: Update
+
+```bash
+pip install --upgrade https://github.com/Panniantong/agent-reach/archive/main.zip
+```
+
+### Step 3: Verify
+
+```bash
+agent-reach version
+agent-reach doctor
+```
+
+Report the new version and channel status to the user. If any channel changed status (was ✅ now ❌), investigate and fix.
+
+### Step 4: Update SKILL.md (if installed)
+
+If the user has the Agent Reach skill installed, update it too:
+
+```bash
+agent-reach install --skill-only
+```
+
+If the `--skill-only` flag is not available, manually copy the latest SKILL.md:
+
+```bash
+python3 -c "
+import importlib.resources, shutil, os
+skill = importlib.resources.files('agent_reach').joinpath('skill', 'SKILL.md').read_text()
+for d in [os.path.expanduser('~/.openclaw/skills/agent-reach'),
+          os.path.expanduser('~/.claude/skills/agent-reach')]:
+    if os.path.isdir(d):
+        with open(os.path.join(d, 'SKILL.md'), 'w') as f:
+            f.write(skill)
+        print(f'✅ Updated: {d}')
+"
+```
+
+Done. Tell the user what version they're now on and how many channels are available.