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/cli/onboard.py

@@ -840,7 +840,7 @@ def _get_channel_info() -> dict[str, tuple[str, type[BaseModel]]]:
                 display_name = getattr(channel_cls, "display_name", name.capitalize())
                 result[name] = (display_name, config_cls)
         except Exception:
-            logger.warning(f"Failed to load channel module: {name}")
+            logger.warning("Failed to load channel module: {}", name)
     return result