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>

internal/extsvc/client.go

@@ -121,13 +121,34 @@ func (c *Client) callOnce(ctx context.Context, method, path string, in, out any)
 		if resp.StatusCode >= 400 {
 			se := &ServiceError{Status: resp.StatusCode}
 			var env struct {
+				// §5 envelope: {"error":{"code":…,"message":…}}.
 				Error struct {
 					Code    string `json:"code"`
 					Message string `json:"message"`
 				} `json:"error"`
+				// Flat envelope, used by the Anypreta atomizer:
+				// {"error_code":…,"message":…}. FastAPI's own validation
+				// errors carry {"detail":…} instead.
+				ErrorCode string          `json:"error_code"`
+				Message   string          `json:"message"`
+				Detail    json.RawMessage `json:"detail"`
 			}
 			if json.Unmarshal(data, &env) == nil {
 				se.Code, se.Message = env.Error.Code, env.Error.Message
+				if se.Code == "" {
+					se.Code = env.ErrorCode
+				}
+				if se.Message == "" {
+					se.Message = env.Message
+				}
+				if se.Message == "" && len(env.Detail) > 0 {
+					se.Message = string(truncate(env.Detail, 200))
+				}
+			}
+			if se.Message == "" {
+				// Never swallow the failure: an unrecognized envelope still
+				// has to reach the logs and the consultant's error toast.
+				se.Message = string(truncate(data, 200))
 			}
 			return se // 4xx is not retryable
 		}