fix(logging): replace stdlib-style exc_info kwargs with loguru opt(exception=)

Loguru has no exc_info kwarg: extra kwargs are str.format() arguments, so
logger.error(f"...{e}", exc_info=True) forces .format() on the rendered
message — any error text containing braces (e.g. a JSON error body) raises
KeyError FROM the log call itself, escaping the surrounding except block and
killing the caller (observed: whole agent turns dying in benchmark runs).
The intended traceback was also never logged, since exc_info is not a loguru
feature.

Batch fix of all 143 sites across 11 files:
  logger.X(msg, exc_info=True) -> logger.opt(exception=True).X(msg)
  (one exc_info=exc site -> opt(exception=exc))
Messages are byte-identical; with the kwarg gone loguru never calls
.format(), so brace-containing f-string messages are inert.

Verified: AST post-conditions per file, py_compile, import smoke of all
modules, behavioral equivalence of the 3 patterns, full unit suite (1549
passed) with a failure set identical to the pristine tree (22 pre-existing,
zero regressions).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Also add pixel-agents to the README acknowledgements.

opc/layer3_agent/external_broker.py

@@ -429,10 +429,9 @@ class ExternalAgentBroker:
                     role_session_id, adapter.agent_type
                 )
             except Exception:
-                logger.debug(
+                logger.opt(exception=True).debug(
                     f"PR6 role adapter-state read failed "
                     f"sid={role_session_id} agent={adapter.agent_type}",
-                    exc_info=True,
                 )
                 entry = None
             if isinstance(entry, dict):
@@ -452,9 +451,8 @@ class ExternalAgentBroker:
                         opc_session_id=role_session_id,
                     )
                 except Exception:
-                    logger.debug(
+                    logger.opt(exception=True).debug(
                         f"External resume restore: get_external_session by role failed for {adapter.agent_type}/{role_session_id}",
-                        exc_info=True,
                     )
                     prior = None
             if prior is None:
@@ -465,9 +463,8 @@ class ExternalAgentBroker:
                         task_id=task.id,
                     )
                 except Exception:
-                    logger.debug(
+                    logger.opt(exception=True).debug(
                         f"External resume restore: get_external_session failed for {adapter.agent_type}/{task.id}",
-                        exc_info=True,
                     )
                     return
             if prior is None:
@@ -1387,7 +1384,7 @@ class ExternalAgentBroker:
             )
             return str(path)
         except Exception:
-            logger.debug("ExternalAgentBroker: failed to write raw external log", exc_info=True)
+            logger.opt(exception=True).debug("ExternalAgentBroker: failed to write raw external log")
             return ""
 
     def _enrich_structured_result_artifacts(
@@ -2231,8 +2228,7 @@ class ExternalAgentBroker:
                         token_record,
                     )
                 except Exception:
-                    logger.debug(
+                    logger.opt(exception=True).debug(
                         f"PR6 role adapter-state write failed "
                         f"sid={role_session_id} agent={adapter.agent_type}",
-                        exc_info=True,
                     )