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>

.dockerignore

@@ -0,0 +1,13 @@
+# Container-owned data: unreadable by the build user, and stat'ing it fails the
+# context walk outright.
+.volumes/
+backups/
+
+.git/
+.env
+.env.*
+!.env.example
+
+# Local working backups.
+.bak-*/
+**/.bak-*/