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/features/boulder-state/storage.ts

@@ -186,7 +186,7 @@ export function getPlanProgress(planPath: string): PlanProgress {
     return {
       total,
       completed,
-      isComplete: total === 0 || completed === total,
+      isComplete: total > 0 && completed === total,
     }
   } catch {
     return { total: 0, completed: 0, isComplete: true }