Config tests that unset a variable now restore it when they end, and
every test that builds a Config first unsets every variable the config
reads, so a value exported in the developer's shell cannot change the
result. TestEnvPort checks that the bad value appears in its errors.
A PORT of zero or below now wraps ErrInvalidPort, as one above 65535
already did.
The README configuration table and the Settings page now say that a
RETENTION_SWEEP_INTERVAL that does not parse, or is zero or negative,
fails startup.
Model: opus-5-5