fix(reddit): add User-Agent to doctor health check to avoid false 403

Reddit API requires a non-empty User-Agent header per its API rules.
Without it, the JSON endpoint returns 403 even when Reddit is reachable,
causing doctor to falsely report the channel as unavailable.

Changes:
- Add _reddit_reachable() helper that probes Reddit JSON API with UA
- doctor check() now distinguishes 'actually unreachable' from 'no proxy'
- Users on home IPs get 'ok' when Reddit is reachable; warn only on real failure

Fixes #168

agent_reach/channels/reddit.py

@@ -1,9 +1,24 @@
 # -*- coding: utf-8 -*-
-"""Reddit — check if proxy and credentials are configured."""
+"""Reddit — check connectivity and proxy configuration."""
 
 import os
+import urllib.request
 from .base import Channel
 
+_UA = "agent-reach/1.0"
+_TIMEOUT = 10
+
+
+def _reddit_reachable() -> bool:
+    """Return True if Reddit JSON API responds with 200 (带 User-Agent)."""
+    url = "https://www.reddit.com/r/linux.json?limit=1"
+    req = urllib.request.Request(url, headers={"User-Agent": _UA})
+    try:
+        with urllib.request.urlopen(req, timeout=_TIMEOUT) as resp:
+            return resp.status == 200
+    except Exception:
+        return False
+
 
 class RedditChannel(Channel):
     name = "reddit"
@@ -20,7 +35,10 @@ class RedditChannel(Channel):
         proxy = (config.get("reddit_proxy") if config else None) or os.environ.get("REDDIT_PROXY")
         if proxy:
             return "ok", "代理已配置，可读取帖子。搜索走 Exa"
+        # 实际探测连通性（带 User-Agent，符合 Reddit API 要求）
+        if _reddit_reachable():
+            return "ok", "直连可用（JSON API 响应正常）。搜索走 Exa"
         return "warn", (
-            "无代理。服务器 IP 可能被 Reddit 封锁。配置代理：\n"
+            "无代理且 Reddit JSON API 无响应。服务器 IP 可能被封锁。配置代理：\n"
             "  agent-reach configure proxy http://user:pass@ip:port"
         )