fix: add missing Zod schema and type definitions for timeoutSeconds

- Zod schema: Add timeoutSeconds to HeartbeatSchema (src/config/zod-schema.agent-runtime.ts)
- TypeScript types: Add timeoutSeconds to heartbeat config with documentation (src/config/types.agent-defaults.ts)
- Runtime: Use conditional spread pattern for heartbeatModelOverride to avoid undefined in object (src/infra/heartbeat-runner.ts)
- Fix duplicate includeReasoning field in HeartbeatSchema

Addresses Greptile review feedback on PR #48568:
1. Missing Zod schema update — configs with timeoutSeconds would be rejected
2. Missing TypeScript type definition — compilation would fail
3. heartbeatModelOverride: undefined explicitly included when only timeout is set

src/config/types.agent-defaults.ts

@@ -269,6 +269,14 @@ export type AgentDefaultsConfig = {
      * Default: false (only the final heartbeat payload is delivered).
      */
     includeReasoning?: boolean;
+    /**
+     * Max timeout in seconds for a heartbeat embedded run.
+     * Allows heartbeats to fail fast (e.g., 60s) when a model hangs,
+     * without affecting the global agents.defaults.timeoutSeconds (default 600s).
+     *
+     * Default: undefined (inherits agents.defaults.timeoutSeconds).
+     */
+    timeoutSeconds?: number;
   };
   /** Max concurrent agent runs across all conversations. Default: 1 (sequential). */
   maxConcurrent?: number;

src/config/zod-schema.agent-runtime.ts

@@ -35,6 +35,7 @@ export const HeartbeatSchema = z
     suppressToolErrorWarnings: z.boolean().optional(),
     lightContext: z.boolean().optional(),
     isolatedSession: z.boolean().optional(),
+    timeoutSeconds: z.number().int().positive().optional(),
   })
   .strict()
   .superRefine((val, ctx) => {

src/infra/heartbeat-runner.ts

@@ -714,7 +714,7 @@ export async function runHeartbeatOnce(opts: {
       heartbeatModelOverride || timeoutOverrideSeconds !== undefined
         ? {
             isHeartbeat: true,
-            heartbeatModelOverride,
+            ...(heartbeatModelOverride !== undefined && { heartbeatModelOverride }),
             suppressToolErrorWarnings,
             bootstrapContextMode,
             ...(timeoutOverrideSeconds !== undefined && { timeoutOverrideSeconds }),