refactor(model-fallback): fully encapsulate session state in factory closure

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/tools/delegate-task/sync-task.ts

@@ -9,7 +9,6 @@ import { SessionCategoryRegistry } from "../../shared/session-category-registry"
 import { formatDuration } from "./time-formatter"
 import { formatDetailedError } from "./error-formatting"
 import { syncTaskDeps, type SyncTaskDeps } from "./sync-task-deps"
-import { setSessionFallbackChain, clearSessionFallbackChain } from "../../hooks/model-fallback/hook"
 import { retrySyncPromptWithFallbacks } from "./sync-task-fallback"
 import { buildTaskMetadataBlock } from "../../features/tool-metadata-store/task-metadata-contract"
 import { resolveMetadataModel } from "./resolve-metadata-model"
@@ -81,7 +80,7 @@ export async function executeSyncTask(
     subagentSessions.add(sessionID)
     syncSubagentSessions.add(sessionID)
     setSessionAgent(sessionID, agentToUse)
-    setSessionFallbackChain(sessionID, fallbackChain)
+    executorCtx.modelFallbackControllerAccessor?.setSessionFallbackChain(sessionID, fallbackChain)
 
     if (args.category) {
       SessionCategoryRegistry.register(sessionID, args.category)
@@ -237,7 +236,7 @@ ${buildTaskMetadataBlock({
     if (syncSessionID) {
       subagentSessions.delete(syncSessionID)
       syncSubagentSessions.delete(syncSessionID)
-      clearSessionFallbackChain(syncSessionID)
+      executorCtx.modelFallbackControllerAccessor?.clearSessionFallbackChain(syncSessionID)
       SessionCategoryRegistry.remove(syncSessionID)
     }
   }