refactor: extract model selection logic from delegate-task into focused modules

- Create available-models.ts for model availability checking
- Create model-selection.ts for category-to-model resolution logic
- Update category-resolver, subagent-resolver, and sync modules to import
  from new focused modules instead of monolithic sources

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

@@ -1,6 +1,6 @@
 import type { DelegateTaskArgs, OpencodeClient } from "./types"
 import { isPlanFamily } from "./constants"
-import { promptSyncWithModelSuggestionRetry } from "../../shared"
+import { promptSyncWithModelSuggestionRetry } from "../../shared/model-suggestion-retry"
 import { formatDetailedError } from "./error-formatting"
 
 export async function sendSyncPrompt(