fix(docker): correct the timezone claim — UTC is the tell, not a country mismatch

The previous commit documented the clock-zone requirement as "the zone
must match the egress IP's country". Re-measuring against the deployment
case shows that is wrong.

The original inference came from reading the host's /etc/timezone
(Asia/Bangkok) and assuming the egress IP was Thai. It is not: this host
egresses from an Indonesian IP. Asia/Bangkok cleared the challenge not
because it matched a country but because it is simply not UTC, and the
two share +07, which hid the distinction.

Measured 2026-08-08, identical container, one Indonesian egress IP:

  TZ=UTC               never cleared (60s, twice)
  TZ=Asia/Jakarta      cleared in 4s
  TZ=America/New_York  cleared in 4s

America/New_York matches neither the country nor the offset nor the
hemisphere, and clears just as fast. So a UTC clock is itself the bot
signal - Cloudflare scores it as the datacenter default - and any real
zone satisfies the check.

This makes the knob considerably less fragile than documented: BROWSER_TZ
needs a plausible zone, not a geolocated one, and a deployment that moves
region does not have to keep it in sync. Comments in chrome/entrypoint.sh
and docker-compose.yml, the hard constraint in AGENTS.md, and the PR
description are corrected accordingly.

BROWSER_TZ is also documented in .env.example for the first time, which
is the file an operator actually copies - the setting decides whether
kagane works at all, and a UTC server (the common case) is exactly the
one that fails with it unset.

Re-verified against the shipped image with TZ=Asia/Jakarta:
TestSmokeKaganeImage PASS (5.00s, 56710 bytes of image/webp),
TestSmokeKaganeGet PASS (1.24s, status 200).

.env.example

@@ -90,3 +90,16 @@ DISCORD_REDIRECT_URI=
 # HTTP handler 500s any /json/version request whose Host header isn't an IP or
 # "localhost", which silently breaks every kagane poll.
 # BROWSER_WS_URL=ws://172.28.0.10:9222
+
+# Clock zone the headless browser reports. A UTC clock is itself the bot
+# signal — Cloudflare treats it as the datacenter default — and kagane's
+# challenge then never clears. Measured 2026-08-08, identical container, one
+# Indonesian egress IP: UTC never cleared in 60s (twice); Asia/Jakarta and
+# America/New_York both cleared in 4s. So any real zone works; it does not
+# have to match the IP's country, it just must not be UTC.
+#
+# Unset falls back to the host's /etc/timezone, which is a real zone whenever
+# the host clock is set to local time. Set this when the host runs UTC — a UTC
+# server is exactly the case that fails. Only the browser sidecar reads it;
+# the backend keeps its UTC clock.
+# BROWSER_TZ=Asia/Jakarta