refactor(gui): todos ride the generic projection pair; ConversationSnapshot evacuated

TodoDock reads useProjection('todos') (whole list or null pre-first-write;
absent renders nothing) and declare-merges the todos key through the pure-type
outlet — the identical member the tool-todo host unit owns, drift rejected by
any program holding both. The core Session's todos field, its todo/write case,
and the snapshot member retire; the client folds nothing. Session specs for
the retired client fold move to the value-store spec's seq coverage; snapshot
literals across component specs drop the field.

packages/client/ui-conversation/tests/chat-code-subcalls.spec.tsx

@@ -56,7 +56,7 @@ function snapshotWith(
 ): ConversationSnapshot {
   return {
     sessionId: SID, nodes, foldDegraded: false, partial: null, runningCalls, codeDispatches,
-    pending: [], queue: [], todos: [], running: runningCalls.length > 0, composerPhase: 'active', removed: false,
+    pending: [], queue: [], running: runningCalls.length > 0, composerPhase: 'active', removed: false,
     openState: 'open', openError: null,
     hasMore: false, loadingOlder: false, promptError: null, blank: false, lastAgentError: null,
   }