fix: propagate variant field in all promptAsync continuation paths (#3081)

All 5 continuation paths now send variant as top-level body field:
- boulder-continuation-injector.ts
- ralph-loop/continuation-prompt-injector.ts
- todo-continuation-enforcer/continuation-injection.ts
- unstable-agent-babysitter-hook.ts
- session-recovery/resume.ts

Plus type/helper updates in atlas, todo-continuation-enforcer,
unstable-agent-babysitter, and session-recovery.

TDD: 18 regression tests added, all pass. tsc clean.

src/hooks/atlas/recent-model-resolver.ts

@@ -40,7 +40,14 @@ export async function resolveRecentPromptContextForSession(
       const model = info?.model
       const tools = normalizePromptTools(info?.tools)
       if (model?.providerID && model?.modelID) {
-        return { model: { providerID: model.providerID, modelID: model.modelID }, tools }
+        return {
+          model: {
+            providerID: model.providerID,
+            modelID: model.modelID,
+            ...(model.variant ? { variant: model.variant } : {}),
+          },
+          tools,
+        }
       }
 
       if (info?.providerID && info?.modelID) {
@@ -63,7 +70,14 @@ export async function resolveRecentPromptContextForSession(
   if (!model?.providerID || !model?.modelID) {
     return { tools }
   }
-  return { model: { providerID: model.providerID, modelID: model.modelID }, tools }
+  return {
+    model: {
+      providerID: model.providerID,
+      modelID: model.modelID,
+      ...(model.variant ? { variant: model.variant } : {}),
+    },
+    tools,
+  }
 }
 
 export async function resolveRecentModelForSession(