refactor(logging): preserve tracebacks and add channel context

- Preserve tracebacks: logger.error in except blocks → logger.exception
- Channel context: BaseChannel injects self.logger = logger.bind(channel=name)
- Third-party bridge: redirect_lib_logging() replaces ad-hoc stdlib-to-loguru bridges
- Log levels: network timeouts downgraded from ERROR → WARNING
- Fix --verbose flag to actually work with loguru (set handler to DEBUG)

nanobot/agent/tools/web.py

@@ -500,10 +500,10 @@ class WebFetchTool(Tool):
                 "untrusted": True, "text": text,
             }, ensure_ascii=False)
         except httpx.ProxyError as e:
-            logger.error("WebFetch proxy error for {}: {}", url, e)
+            logger.exception("WebFetch proxy error for {}", url)
             return json.dumps({"error": f"Proxy error: {e}", "url": url}, ensure_ascii=False)
         except Exception as e:
-            logger.error("WebFetch error for {}: {}", url, e)
+            logger.exception("WebFetch error for {}", url)
             return json.dumps({"error": str(e), "url": url}, ensure_ascii=False)
 
     def _to_markdown(self, html_content: str) -> str: