fix(frank-zealot): serve /metrics so Prometheus can see the endpoint

The scrape has never worked: lastError is "server returned HTTP status 501 Not
Implemented" on http://host.containers.internal:7771/metrics, and the oldest
retained sample of up{job="frank-zealot"} (2026-10-11) is already 0 -- the target
has been red across four payload generations while /health answered 200 the whole
time, keeping the critical LlmEndpointDown alert permanently active. llama.cpp
gates /metrics behind --metrics; halogen serves it natively, which is why the
asymmetry looked like a 3090-only problem.

METRICS=1 in the shipped env, --metrics reaches the argv through EXTRA_ARGS, and
the seam asserts both directions (default ships the flag, METRICS=0 strips it).
Diagnosed via /api/v1/targets lastError rather than the alert list, which is also
the lesson: Alertmanager here has no persistence, so a reboot re-stamps startsAt
and a weeks-old fault looks fresh.

scripts/frank-zealot.env

@@ -70,3 +70,12 @@ NGL=99
 # and graph capture land in the journal, which is what the VRAM-fit forensics
 # read. Levels above 4 flood the journal.
 VERBOSITY=4
+
+# Serve /metrics. llama.cpp answers 501 without --metrics, and Dankclaw's
+# Prometheus scrapes /metrics on every fleet endpoint — a 501 reads as DOWN, so
+# this service has held the critical LlmEndpointDown alert for as long as the
+# metric has history (verified 2026-10-18: the oldest retained sample of
+# up{job="frank-zealot"}, 2026-10-11, is already 0; scrapeUrl
+# http://host.containers.internal:7771/metrics, lastError "server returned HTTP
+# status 501 Not Implemented"). 0 opts out and re-breaks the scrape.
+METRICS=1