simplify: install.md 只说目标，不说怎么修——Agent 自己会

install.md 核心指令：
- 目标：所有渠道都 ✅
- 遇到问题自己修，不要停
- 只有需要用户提供的东西（密码/cookie/key）才问用户
- 最后跑 doctor 汇报结果

agent_reach/cli.py

@@ -220,9 +220,7 @@ def _cmd_install(args):
 
     if ok < total:
         print()
-        print("🤖 Note for AI Agents: You have the ability to fix the issues above.")
-        print("   Try alternative install methods, check error logs, or search for solutions.")
-        print("   Only ask the user for things you genuinely can't do yourself (API keys, cookies, passwords).")
+        print("🤖 Some channels are not active yet. If you're an AI Agent, try to fix them yourself — then run `agent-reach doctor` again.")
 
 
 def _install_system_deps():