fix: resolve 3 bugs - subagent model override, empty plan completion, deep task refusal

- #2741: Pass inheritedModel as fallback in subagent-resolver when user hasn't
  configured an override, ensuring custom provider models take priority
- #2648: Fix getPlanProgress to treat plans with 0 checkboxes as incomplete
  instead of complete (total > 0 && completed === total)
- #2779: Relax Hephaestus single-task guard to accept multi-step sub-tasks
  from Atlas delegation, only rejecting genuinely independent tasks

Fixes #2741, fixes #2648, fixes #2779

src/tools/delegate-task/subagent-resolver.ts

@@ -17,7 +17,8 @@ export async function resolveSubagentExecution(
   args: DelegateTaskArgs,
   executorCtx: ExecutorContext,
   parentAgent: string | undefined,
-  categoryExamples: string
+  categoryExamples: string,
+  inheritedModel?: string
 ): Promise<{ agentToUse: string; categoryModel: { providerID: string; modelID: string; variant?: string } | undefined; fallbackChain?: FallbackEntry[]; error?: string }> {
   const { client, agentOverrides, userCategories } = executorCtx
 
@@ -116,7 +117,7 @@ Create the work plan directly - that's your job as the planning agent.`,
         : undefined
 
       const resolution = resolveModelForDelegateTask({
-        userModel: agentOverride?.model,
+        userModel: agentOverride?.model ?? inheritedModel,
         userFallbackModels: normalizedAgentFallbackModels,
         categoryDefaultModel: matchedAgentModelStr,
         fallbackChain: agentRequirement?.fallbackChain,