feat: add Pi subagent scheduler and child runtime

tests/unit/pi-session-projector.test.ts

@@ -33,6 +33,57 @@ function baseSnapshot(): ConversationSnapshot {
 const zeroCost = { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 };
 
 describe('Pi session projector', () => {
+  it('hydrates stable subagent nodes and bounds unknown detail versions', async () => {
+    const snapshot = await projectPiSessionSnapshot({
+      snapshot: baseSnapshot(),
+      workerGeneration: 1,
+      state: { sessionId: 'session-a', isStreaming: false, isCompacting: false },
+      entries: {
+        leafId: 'entry-unknown-result',
+        entries: [
+          {
+            type: 'message', id: 'entry-assistant', parentId: null,
+            message: {
+              role: 'assistant', stopReason: 'toolUse', usage: { input: 1, output: 1 },
+              content: [
+                { type: 'toolCall', id: 'call-known', name: 'subagent', arguments: {} },
+                { type: 'toolCall', id: 'call-unknown', name: 'subagent', arguments: {} },
+              ],
+            },
+          },
+          {
+            type: 'message', id: 'entry-known-result', parentId: 'entry-assistant',
+            message: {
+              role: 'toolResult', toolCallId: 'call-known', toolName: 'subagent', content: [],
+              details: {
+                schema: 'subagent.v1', dispatchId: 'dispatch-a', mode: 'single',
+                tasks: [{
+                  taskId: 'task-a', agentId: 'agent-a', toolProfile: 'coding', status: 'complete',
+                }],
+              },
+            },
+          },
+          {
+            type: 'message', id: 'entry-unknown-result', parentId: 'entry-known-result',
+            message: {
+              role: 'toolResult', toolCallId: 'call-unknown', toolName: 'subagent', content: [],
+              details: { schema: 'subagent.v9', raw: 'RAW_SESSION_SECRET' },
+            },
+          },
+        ],
+      },
+    });
+    expect(snapshot.nodes).toContainEqual(expect.objectContaining({
+      kind: 'subagent', id: 'subagent:dispatch-a',
+      details: expect.objectContaining({ schema: 'subagent.v1' }),
+    }));
+    expect(JSON.stringify(snapshot)).not.toContain('RAW_SESSION_SECRET');
+    expect(snapshot.nodes).toContainEqual(expect.objectContaining({
+      kind: 'tool', toolCallId: 'call-unknown',
+      output: [expect.objectContaining({ text: 'Subagent details are unavailable for this version.' })],
+    }));
+  });
+
   it('hydrates only the authoritative active leaf path', async () => {
     const snapshot = await projectPiSessionSnapshot({
       snapshot: baseSnapshot(),