Every time in UTC, and every day as YYYY-MM-DD (closes #313)

AbuseIPDB's daily count keeps its day as a civil.Date, taken from the UTC
clock and written to reputation.json as "YYYY-MM-DD"; it was a time.Time
at midnight. Every time read from a state file is converted to UTC, so a
time an admin writes with an offset is written back in UTC.
TestMainRunsWAF runs sww waf with TZ 14 hours ahead of UTC and checks
that the process line, the request line and clients.json are in UTC.
SPEC.md states the rule near its start; AGENTS.md makes it an iron rule.

Model: opus-5-5

AGENTS.md

@@ -37,3 +37,6 @@ These are the rules for agents working on this repository.
         - `192.168.0.0/16`.
     - `SPEC.md` states that default.
     - Forwarded headers count only when the peer is a trusted proxy.
+- Every time is in UTC, whatever `TZ` or the host's time zone says.
+    - Every day is a date written `YYYY-MM-DD`, never a time.
+    - `SPEC.md` states this near its start.