fix(state): empty slice plan stays in planning, not summarizing (#454)

A plan file with zero tasks caused `find(t => !t.done)` to return
undefined, which was treated as "all tasks done" → summarizing phase.
Now requires `tasks.length > 0` before entering summarizing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/resources/extensions/gsd/state.ts

@@ -470,7 +470,7 @@ async function _deriveStateImpl(basePath: string): Promise<GSDState> {
   };
   const activeTaskEntry = slicePlan.tasks.find(t => !t.done);
 
-  if (!activeTaskEntry) {
+  if (!activeTaskEntry && slicePlan.tasks.length > 0) {
     // All tasks done but slice not marked complete
     return {
       activeMilestone,
@@ -491,6 +491,27 @@ async function _deriveStateImpl(basePath: string): Promise<GSDState> {
     };
   }
 
+  // Empty plan — no tasks defined yet, stay in planning phase
+  if (!activeTaskEntry) {
+    return {
+      activeMilestone,
+      activeSlice,
+      activeTask: null,
+      phase: 'planning',
+      recentDecisions: [],
+      blockers: [],
+      nextAction: `Slice ${activeSlice.id} has a plan file but no tasks. Add tasks to the plan.`,
+      activeBranch: activeBranch ?? undefined,
+      registry,
+      requirements,
+      progress: {
+        milestones: milestoneProgress,
+        slices: sliceProgress,
+        tasks: taskProgress,
+      },
+    };
+  }
+
   const activeTask: ActiveRef = {
     id: activeTaskEntry.id,
     title: activeTaskEntry.title,

src/resources/extensions/gsd/tests/derive-state.test.ts

@@ -651,6 +651,41 @@ Continue from step 2.
     }
   }
 
+  // ─── Empty plan (zero tasks) stays in planning, not summarizing (#454) ──
+  console.log('\n=== empty plan → planning (not summarizing) ===');
+  {
+    const base = createFixtureBase();
+    try {
+      writeRoadmap(base, 'M001', `---
+id: M001
+title: "Test"
+---
+# M001: Test
+## Vision
+Test
+## Success Criteria
+- Done
+## Slices
+- [ ] **S01: Empty slice** \`risk:low\` \`depends:[]\`
+  > Test
+## Boundary Map
+_None_
+`);
+      writePlan(base, 'M001', 'S01', `---
+slice: S01
+---
+# S01 Plan
+## Tasks
+`);
+      const state = await deriveState(base);
+      assertEq(state.phase, 'planning', 'empty plan stays in planning');
+      assertEq(state.activeSlice?.id, 'S01', 'active slice is S01');
+      assertEq(state.activeTask, null, 'no active task');
+    } finally {
+      cleanup(base);
+    }
+  }
+
   report();
 }