Log caller (file:line) to help debugging errors

[akrem-chabchoub]

Summary

Add optional file/line field to log lines, click it in the terminal to jump to that spot in the editor.

Motivation

Easier debugging: see where log lines come from without grepping. Feature exists in pkg/log, not exposed.

Implementation

Enable caller in logger; config/flag to turn on/off and pick category (error vs all). Reuse WithCaller, optionally WithCallerFunc. .

Drawbacks

runtime.Caller() costs CPU per line; more log bytes. Default off, category=error only to limit.