DEBUG turns debug logging on for any non-empty value (closes #159)

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

TODO.md

@@ -21,6 +21,12 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-19: `DEBUG` unset or empty leaves debug logging off, and any other
+  value, `false` and `0` included, turns it on, as the code style guide says
+  ([#159](https://git.eeqj.de/sneak/netwatch/issues/159)). No value of it stops
+  the start any more; before, only `1`, `t`, `T`, `TRUE`, `true`, `True`, `0`,
+  `f`, `F`, `FALSE`, `false` and `False` were accepted. `README.md` no longer
+  gives `false` as its default
 - 2026-10-19: check builds keep Go's modules and compiled packages between runs
   ([#118](https://git.eeqj.de/sneak/netwatch/issues/118)), on sneak's rulings in
   [prompts#113](https://git.eeqj.de/sneak/prompts/issues/113) and