fix: birdx → bird CLI (npm @steipete/bird)

birdx 从来不是 PyPI 包，pip install birdx 必然失败。
实际工具是 npm 包 @steipete/bird，一个 Twitter GraphQL CLI。

变更：
- 安装器改用 npm install -g @steipete/bird
- twitter.py 直接调 bird，通过环境变量传 AUTH_TOKEN/CT0
- 兼容已有的 birdx wrapper（shutil.which 回退）
- 更新所有文档引用
- 重写 setup-twitter.md 指南

docs/install.md

@@ -23,7 +23,7 @@ pip install https://github.com/Panniantong/agent-reach/archive/main.zip
 agent-reach install --env=auto
 ```
 
-This auto-installs system dependencies (gh CLI, Node.js, mcporter, birdx), configures Exa search, detects environment, and tests all channels.
+This auto-installs system dependencies (gh CLI, Node.js, mcporter, bird), configures Exa search, detects environment, and tests all channels.
 
 **Read the output carefully**, then run: