feat(atomize): serve Subdivide via Anypreta Issue Atomizer

Add an optional remote atomization backend (the Anypreta "Issue Atomizer"
API) for the Subdivide flow, selected by ATOMIZER_REMOTE_BASE_URL /
ATOMIZER_REMOTE_API_KEY. When set, POST /v1/atomize is served by the remote
/v1/atomize-issue endpoint; its two-level issue→features→tasks model is mapped
onto our one-level task→children model by treating each Feature as a child and
its `estimation` as the effort coefficient (renormalized to sum to 1, even
split as a reported fallback when estimations are absent).

Extend and Summarize have no counterpart in the remote API and always stay on
ATOMIZER_BASE_URL, so the §5.1 service must keep running; readiness now probes
both backends. The mandatory `system` object is derived from the customer,
with a generic component tree (an empty tree makes the service return zero
features).

extsvc also learns the remote's flat error envelope ({error_code,message} and
FastAPI {detail}) and never swallows an unrecognized 4xx body.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cmd/app/main.go

@@ -98,6 +98,13 @@ func run() error {
 		}
 		return cfg.AtomizerBaseURL
 	}, cfg.AtomizerToken, cfg.AtomizerTimeout)
+	if cfg.AtomizerRemoteBaseURL != "" {
+		// Subdivide is served by the Anypreta Issue Atomizer; Extend and
+		// Summarize have no counterpart there and stay on ATOMIZER_BASE_URL.
+		atomClient.UseRemote(cfg.AtomizerRemoteBaseURL, cfg.AtomizerRemoteAPIKey, cfg.AtomizerTimeout)
+		log.Info("atomizer: subdivide served by remote backend",
+			"url", cfg.AtomizerRemoteBaseURL, "extend_summarize", cfg.AtomizerBaseURL)
+	}
 	srv.SetAtomizerInfo(atomClient)
 	srv.AddReadinessCheck(httpx.ReadinessCheck{Name: "atomizer", Probe: atomClient.Healthy})