fix(boulder-state): treat plans without checkboxes as incomplete (fixes #2648)

GPT/Gemini Prometheus plans sometimes lack markdown checkboxes.
Previously getPlanProgress() returned isComplete=true for 0/0,
causing /start-work to skip Atlas execution.

Now total=0 correctly returns isComplete=false so start-work
detects the invalid plan format.

🤖 Generated with assistance of [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/features/boulder-state/storage.ts

@@ -133,7 +133,7 @@ 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 }