fix(config): add timeoutSeconds support to agents.defaults.heartbeat

The heartbeat config schema was missing the timeoutSeconds field that was
documented in heartbeat.md. This caused config validation to fail when users
set timeoutSeconds under agents.defaults.heartbeat.

Changes:
- Add timeoutSeconds to HeartbeatSchema (z.number().int().positive().optional())
- Add timeoutSeconds type definition in AgentDefaultsConfig
- Add JSDoc comment for the new field

Fixes #64437

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

src/config/types.agent-defaults.ts

@@ -299,6 +299,8 @@ export type AgentDefaultsConfig = {
     ackMaxChars?: number;
     /** Suppress tool error warning payloads during heartbeat runs. */
     suppressToolErrorWarnings?: boolean;
+    /** Run timeout in seconds for heartbeat agent turns. */
+    timeoutSeconds?: number;
     /**
      * If true, run heartbeat turns with lightweight bootstrap context.
      * Lightweight mode keeps only HEARTBEAT.md from workspace bootstrap files.

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

@@ -38,6 +38,7 @@ export const HeartbeatSchema = z
     includeSystemPromptSection: z.boolean().optional(),
     ackMaxChars: z.number().int().nonnegative().optional(),
     suppressToolErrorWarnings: z.boolean().optional(),
+    timeoutSeconds: z.number().int().positive().optional(),
     lightContext: z.boolean().optional(),
     isolatedSession: z.boolean().optional(),
   })