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>

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.