* fix(agents): do not hide invalid config with file fallback
* test(agents): cover invalid on-disk config fallback
* fix(agents): resolve stores off the event loop
* fix(agents): distinguish missing nested config from main config
* fix(agents): reject missing explicit config path
* test(agents): isolate config fallback test
* test(agents): isolate router blocking IO coverage
* test(agents): pin malformed config.yaml parse-error propagation
An unparseable config.yaml used to be swallowed by the broad
except Exception and silently downgrade to FileAgentStore. The
narrowed except FileNotFoundError already propagates
yaml.ParserError/ScannerError; pin that contract with a real
on-disk config instead of monkeypatched get_app_config.
fb722770e4