fix: 4 bugs found in full E2E usage testing

Bug 1: Reddit 403 due to short User-Agent in proxy test
  Root cause: configure proxy test used 'Mozilla/5.0' (too short, Reddit rejects)
  Fix: use full UA string matching the channel implementation

Bug 2: Reddit URL parsing broke on trailing slashes
  Root cause: url.rstrip('/') + '.json' mangled URLs with query params
  Fix: properly parse URL, clean path, reconstruct json URL

Bug 3: Reddit 403 without proxy showed raw HTTP error
  Root cause: no error handling for 403/429 responses
  Fix: friendly message suggesting proxy setup + Exa search alternative

Bug 4: XiaoHongShu without cookie showed Jina Reader 451 error
  Root cause: fallback to Jina Reader which can't access XHS (legal block)
  Fix: show clear message about needing cookies with setup instructions

Bug 5: Empty search query caused raw 422 API error
  Root cause: no input validation before API call
  Fix: check for empty query, show friendly message

All 36 unit tests passing.

agent_eyes/cli.py

@@ -311,7 +311,7 @@ def _cmd_configure(args):
             import requests
             resp = requests.get(
                 "https://www.reddit.com/r/test.json?limit=1",
-                headers={"User-Agent": "Mozilla/5.0"},
+                headers={"User-Agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36"},
                 proxies={"http": value, "https": value},
                 timeout=10,
             )
@@ -539,9 +539,13 @@ async def _cmd_read(args):
 async def _cmd_search(args):
     from agent_eyes.core import AgentEyes
     eyes = AgentEyes()
-    query = " ".join(args.query)
+    query = " ".join(args.query).strip()
     num = args.num
 
+    if not query:
+        print("Please provide a search query.", file=sys.stderr)
+        sys.exit(1)
+
     try:
         if args.command == "search":
             results = await eyes.search(query, num_results=num)