Document the majormail spam-routing failure (2026-06-06): a cleanup
header_checks REDIRECT keyed on the milter-added X-Spam-Flag never fired for
real inbound mail (only locally-injected), so spam kept reaching the inbox.
Fix is to route in Sieve at delivery (after the milter), with a redirect +
loop guard. Includes the 'local-injection tests lie' warning.
