Investigates a test failure, bug, or unexpected behavior to find its root cause before proposing any fix - reproduce the issue, trace the data flow, form and test one hypothesis at a time, then implement and verify the fix. This is the debugging stage inside the delivery-flow workflow, entered by requests like "debug this failure", "why is this test failing", "find the root cause", or "this bug keeps coming back" once delivery-flow has handed off a bug. Do not activate directly for a standalone bug report that has not gone through delivery-flow first.