refactor(artwork): drop the unreachable breaker branch from the tracing gate

--live wires the tracing gate straight to passthroughGate, so errBreakerOpen can
never reach it; the test only passed by injecting a fake gate.

core/artwork/artwork.go

@@ -402,6 +402,8 @@ type Resolver struct {
 func NewTracingResolver(ds model.DataStore, ag *agents.Agents, ffm ffmpeg.FFmpeg, t *ChainTrace, live bool) *Resolver {
 	gate := offlineGate(t)
 	if live {
+		// A diagnostic must show the provider's real answer, and one item is at most one call
+		// per agent, so --live deliberately bypasses the rate limiter and circuit breaker.
 		gate = tracingGate(t, passthroughGate)
 	}
 	return &Resolver{inner: newResolver(ds, ag, ffm, gate), trace: t}

core/artwork/trace.go

@@ -79,8 +79,6 @@ func tracingGate(t *chainTrace, inner gateFunc) gateFunc {
 		switch {
 		case r != nil:
 			t.add(traceStep{Candidate: candidate, Outcome: OutcomeHit, Detail: path})
-		case errors.Is(err, errBreakerOpen):
-			t.add(traceStep{Candidate: candidate, Outcome: OutcomeSkipped, Detail: "circuit breaker open"})
 		case isTransientExternal(err):
 			t.add(traceStep{Candidate: candidate, Outcome: OutcomeError, Detail: err.Error()})
 		default:

core/artwork/trace_test.go

@@ -158,16 +158,6 @@ var _ = Describe("external gate tracing", func() {
 		Expect(t.Steps()[0].Detail).To(ContainSubstring("429"))
 	})
 
-	It("records skipped when the breaker is open", func() {
-		t := &chainTrace{}
-		open := func(string, func() (io.ReadCloser, string, error)) (io.ReadCloser, string, error) {
-			return nil, "", errBreakerOpen
-		}
-		_, _, _ = tracingGate(t, open)("apple-music", hit)
-		Expect(t.Steps()[0].Outcome).To(Equal(OutcomeSkipped))
-		Expect(t.Steps()[0].Detail).To(ContainSubstring("circuit breaker"))
-	})
-
 	It("never calls the agent in offline mode", func() {
 		t := &chainTrace{}
 		called := false