refactor(delegate-task): deduplicate DelegatedModelConfig + registry refactor

- Move DelegatedModelConfig to src/shared/model-resolution-types.ts
- Re-export from delegate-task/types.ts (preserving import paths)
- Replace background-agent/types.ts local duplicate with shared import
- Consolidate model-settings-compatibility.ts registry patterns

src/features/background-agent/types.ts

@@ -1,4 +1,5 @@
 import type { FallbackEntry } from "../../shared/model-requirements"
+import type { DelegatedModelConfig } from "../../shared/model-resolution-types"
 import type { SessionPermissionRule } from "../../shared/question-denied-session-permission"
 
 export type BackgroundTaskStatus =
@@ -25,17 +26,6 @@ export interface TaskProgress {
   lastMessageAt?: Date
 }
 
-type DelegatedModelConfig = {
-  providerID: string
-  modelID: string
-  variant?: string
-  reasoningEffort?: string
-  temperature?: number
-  top_p?: number
-  maxTokens?: number
-  thinking?: { type: "enabled" | "disabled"; budgetTokens?: number }
-}
-
 export interface BackgroundTask {
   id: string
   sessionID?: string