feat(web): use shared user-agent and send X-Keenable-Title

Drop the bespoke nanobot/<version> User-Agent in favor of self.user_agent
for consistency with every other search provider, and add an
X-Keenable-Title: nanobot header so Keenable can attribute traffic.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/tools/test_web_search_tool.py

@@ -143,7 +143,8 @@ async def test_keenable_search(monkeypatch):
     async def mock_post(self, url, **kw):
         assert "keenable" in url
         assert kw["headers"]["X-API-Key"] == "keen-key"
-        assert kw["headers"]["User-Agent"].startswith("nanobot/")
+        assert kw["headers"]["User-Agent"] == "nanobot-search-test"
+        assert kw["headers"]["X-Keenable-Title"] == "nanobot"
         return _response(json={
             "results": [{"title": "Keen", "url": "https://keenable.ai", "description": "short", "snippet": "longer excerpt"}]
         })