fix(config): detect top-level heartbeat as invalid config path (#30894)

Top-level `heartbeat` is not a valid config path but was silently
ignored, causing users to believe heartbeat.model was configured
when it was not. Add a legacy config rule so startup validation
surfaces a clear migration hint pointing to agents.defaults.heartbeat.

Fixes #30894

src/config/config.legacy-config-detection.accepts-imessage-dmpolicy.test.ts

@@ -274,6 +274,15 @@ describe("legacy config detection", () => {
       },
     );
   });
+  it("flags top-level heartbeat as legacy in snapshot", async () => {
+    await withSnapshotForConfig(
+      { heartbeat: { model: "anthropic/claude-3-5-haiku-20241022", every: "30m" } },
+      async (ctx) => {
+        expect(ctx.snapshot.valid).toBe(false);
+        expect(ctx.snapshot.legacyIssues.some((issue) => issue.path === "heartbeat")).toBe(true);
+      },
+    );
+  });
   it("flags legacy provider sections in snapshot", async () => {
     await withSnapshotForConfig({ whatsapp: { allowFrom: ["+1555"] } }, async (ctx) => {
       expect(ctx.snapshot.valid).toBe(false);

src/config/legacy.rules.ts

@@ -204,4 +204,9 @@ export const LEGACY_CONFIG_RULES: LegacyConfigRule[] = [
     match: (value) => isLegacyGatewayBindHostAlias(value),
     requireSourceLiteral: true,
   },
+  {
+    path: ["heartbeat"],
+    message:
+      "top-level heartbeat is not a valid config path; use agents.defaults.heartbeat instead.",
+  },
 ];