fix: correct the .cod rationale, and stop world-reading the cache

The comment claimed a bad key returns HTTP 200 with an error body, so
the status code alone proves nothing. Measured against the live API,
that is wrong: a bad key returns 401 and an unknown city 404, and with
curl -f both fail at curl without the .cod check ever running.

The claim came from the polybar script, which ran curl WITHOUT -f and so
genuinely did receive error bodies with a success exit. The check was
ported but not the reason for it. It still earns its place behind -f,
for a 200 whose body is not usable weather, and the comment now says
that instead of something false. The spec carried the same wrong
sentence and is corrected too.

Separately, the cache landed at 644 in a directory where every other
generated file is 600, and the response carries the configured city and
its coordinates. A umask before the write puts it at 600 without a
window where it is briefly readable.

The .cod discrepancy was found by the Task 6 implementer, which noticed
the bad-key test produced a different message than the plan predicted
and said so rather than reporting the expected text as achieved.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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