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.

tests/frank_zealot_unit.bats

@@ -63,6 +63,7 @@ CACHE_TYPE_V=$(sed -nE 's/^CACHE_TYPE_V=(.+)$/\1/p' "$ENV_FILE")
 SPEC_TYPE=$(sed -nE 's/^SPEC_TYPE=(.+)$/\1/p' "$ENV_FILE")
 SPEC_DRAFT_N_MAX=$(sed -nE 's/^SPEC_DRAFT_N_MAX=([0-9]+)$/\1/p' "$ENV_FILE")
 VERBOSITY=$(sed -nE 's/^VERBOSITY=([0-9]+)$/\1/p' "$ENV_FILE")
+METRICS=$(sed -nE 's/^METRICS=([0-9]+)$/\1/p' "$ENV_FILE")
 NGL=$(sed -nE 's/^NGL=([0-9]+)$/\1/p' "$ENV_FILE")
 EOF
     run env FRANK_ZEALOT_ENV_FILE="$envf" nvidia-modprobe=true bash "$SERVE"
@@ -176,6 +177,22 @@ run_stage() {
     [ "$(argv_value --log-verbosity)" = "3" ]
 }
 
+@test "the Prometheus metrics endpoint is served unless opted out" {
+    # Output contract, not a config read: what matters to the operator is that
+    # the engine is exec'd with --metrics, because llama.cpp answers 501 on
+    # /metrics without it and the fleet scrape then reports the endpoint DOWN
+    # (the oldest retained sample of this target's scrape, 2026-10-11, is a 0).
+    serve_argv
+    [ "$status" -eq 0 ]
+    grep -qxF -- '--metrics' <<<"$output"
+
+    # And the opt-out must be real: METRICS=0 ships no /metrics at all.
+    sed -i 's/^METRICS=.*/METRICS=0/' "$STAGE/test.env"
+    run_stage
+    [ "$status" -eq 0 ]
+    ! grep -qxF -- '--metrics' <<<"$output"
+}
+
 @test "the compute-buffer pair follows the env file and stays self-consistent" {
     # The contract is: whatever rung the repo carries is the rung the engine
     # is exec'd with — the number a benchmark attributes a prefill rate to