Observability Setup — Which Backend + One-Line Autolog

"Instrument before you need it. The cheapest debugging session is a trace you already have."

This is an ENHANCE overlay. The local skills [[langsmith]], [[phoenix]] and [[mlflow]] each teach one backend deeply. This skill sits one level up: it answers the question those skills cannot — which one, and how do I turn it on in a single line right now — then hands off to the chosen backend's own skill for depth.

1. 何时激活 (When to activate)

Activate at one of three moments — earlier is always cheaper: