fix: add agent field to readMetadata return mapping

readMetadata() was not returning the `agent` field even though
writeMetadata() persisted it. This meant typed consumers got
undefined for session.agent while raw readers worked fine.

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

packages/core/src/__tests__/session-manager.test.ts

@@ -303,6 +303,16 @@ describe("spawn", () => {
       expect(meta).not.toBeNull();
       expect(meta!["agent"]).toBe("mock-agent");
     });
+
+    it("readMetadata returns agent field (typed SessionMetadata)", async () => {
+      const sm = createSessionManager({ config, registry: registryWithMultipleAgents });
+
+      await sm.spawn({ projectId: "my-app", agent: "codex" });
+
+      const meta = readMetadata(sessionsDir, "app-1");
+      expect(meta).not.toBeNull();
+      expect(meta!.agent).toBe("codex");
+    });
   });
 
   it("validates issue exists when issueId provided", async () => {

packages/core/src/metadata.ts

@@ -97,6 +97,7 @@ export function readMetadata(dataDir: string, sessionId: SessionId): SessionMeta
     pr: raw["pr"],
     summary: raw["summary"],
     project: raw["project"],
+    agent: raw["agent"],
     createdAt: raw["createdAt"],
     runtimeHandle: raw["runtimeHandle"],
     dashboardPort: raw["dashboardPort"] ? Number(raw["dashboardPort"]) : undefined,