fix(delegate-task): reject primary agents in task subagent resolution

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

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

src/tools/delegate-task/fallback-entry-resolution.ts

@@ -0,0 +1,27 @@
+import { findMostSpecificFallbackEntry } from "../../shared/fallback-chain-from-models"
+import type { FallbackEntry } from "../../shared/model-requirements"
+import type { DelegatedModelConfig } from "./types"
+
+export function resolveEffectiveFallbackEntry(input: {
+  categoryModel: DelegatedModelConfig | undefined
+  configuredFallbackChain: FallbackEntry[] | undefined
+  resolution:
+    | { skipped: true }
+    | { fallbackEntry?: FallbackEntry; matchedFallback?: boolean }
+    | undefined
+}): FallbackEntry | undefined {
+  const { categoryModel, configuredFallbackChain, resolution } = input
+
+  const resolutionSkipped = resolution && "skipped" in resolution
+  const resolvedFallbackEntry = resolution && !resolutionSkipped ? resolution.fallbackEntry : undefined
+  const matchedFallback = resolution && !resolutionSkipped ? resolution.matchedFallback === true : false
+
+  if (!matchedFallback || !categoryModel) {
+    return undefined
+  }
+
+  return resolvedFallbackEntry
+    ?? (configuredFallbackChain
+      ? findMostSpecificFallbackEntry(categoryModel.providerID, categoryModel.modelID, configuredFallbackChain)
+      : undefined)
+}