DEBUG was read with strconv.ParseBool, so a value such as "yes" stopped
the start. It now follows the code style guide: unset or empty is off,
and any other value, "false" and "0" included, turns debug logging on.
No value of DEBUG stops the start. The test that "yes" fails is replaced
by one covering unset, empty, "false", "0" and "yes". README.md no
longer gives false as the default, and says which values turn it on.
Model: opus-5-5