fix: install 自动安装 gh CLI + Node.js，加大 mcporter 超时

问题：新服务器上 gh CLI 和 Node.js 未安装，mcporter npm install 60s 超时
修复：
- 新增 _install_system_deps()：自动安装 gh CLI (apt) + Node.js (nodesource)
- mcporter 超时从 60s → 120s
- install.md 更新：去掉过时的 exa-key/xhs-cookie 配置步骤
- install 流程：系统依赖 → mcporter → Exa 配置 → 环境检测 → 测试

docs/install.md

@@ -20,9 +20,10 @@ agent-reach install --env=auto
 ```
 
 This single command does everything automatically:
+- Installs system dependencies (gh CLI, Node.js, mcporter) if missing
+- Configures Exa search (free, no API key needed)
 - Detects local computer vs server
-- Auto-imports browser cookies on local (Twitter, XiaoHongShu, Bilibili)
-- Auto-detects Exa API key if already in environment
+- Auto-imports browser cookies on local (Twitter)
 - Tests all channels
 - Shows what's working and what needs setup
 
@@ -33,20 +34,12 @@ This single command does everything automatically:
 Based on the install output, you may need to configure one or two things.
 Only ask the user for items the install output says are missing.
 
-**If search is not configured:**
-> "Want to unlock web search? Go to https://exa.ai → sign up (free) → send me the API key."
-
-```bash
-agent-reach configure exa-key THE_KEY
-```
-
 **If cookies were not auto-imported (server users):**
-> "To unlock Twitter and XiaoHongShu, install the Cookie-Editor Chrome extension, go to the site, click Export → Header String, and paste it to me."
+> "To unlock Twitter search and posting, install the Cookie-Editor Chrome extension, go to twitter.com, click Export → Header String, and paste it to me."
 > Extension: https://chromewebstore.google.com/detail/cookie-editor/hlkenndednhfkekhgcdicdfddnkalmdm
 
 ```bash
 agent-reach configure twitter-cookies "PASTED_STRING"
-agent-reach configure xhs-cookie "PASTED_STRING"
 ```
 
 **If on server and wants Reddit/Bilibili full access:**
@@ -70,11 +63,9 @@ Report what's active. Users can configure more anytime by asking.
 
 | Command | What it does |
 |---------|-------------|
-| `agent-reach install --env=auto` | Full auto-setup |
+| `agent-reach install --env=auto` | Full auto-setup (installs deps + configures) |
 | `agent-reach doctor` | Show status |
-| `agent-reach configure exa-key KEY` | Unlock search |
-| `agent-reach configure twitter-cookies "..."` | Unlock Twitter |
-| `agent-reach configure xhs-cookie "..."` | Unlock XiaoHongShu |
+| `agent-reach configure twitter-cookies "..."` | Unlock Twitter search + posting |
 | `agent-reach configure proxy URL` | Unlock Reddit + Bilibili (server) |
 | `agent-reach read URL` | Read any URL |
 | `agent-reach search "query"` | Search the web |