feat(heartbeat): add accountId config option for multi-agent routing (#8702)

* feat(heartbeat): add accountId config option for multi-agent routing

Add optional accountId field to heartbeat configuration, allowing
multi-agent setups to explicitly specify which Telegram account
should be used for heartbeat delivery.

Previously, heartbeat delivery would use the accountId from the
session's deliveryContext. When a session had no prior conversation
history, heartbeats would default to the first/primary account
instead of the agent's intended bot.

Changes:
- Add accountId to HeartbeatSchema (zod-schema.agent-runtime.ts)
- Use heartbeat.accountId with fallback to session accountId (targets.ts)

Backward compatible: if accountId is not specified, behavior is unchanged.

Closes #8695

* fix: improve heartbeat accountId routing (#8702) (thanks @lsh411)

* fix: harden heartbeat accountId routing (#8702) (thanks @lsh411)

* fix: expose heartbeat accountId in status (#8702) (thanks @lsh411)

* chore: format status + heartbeat tests (#8702) (thanks @lsh411)

---------

Co-authored-by: m1 16 512 <m116512@m1ui-MacBookAir-2.local>
Co-authored-by: Gustavo Madeira Santana <gumadeiras@gmail.com>

src/config/types.agent-defaults.ts

@@ -182,6 +182,8 @@ export type AgentDefaultsConfig = {
     target?: "last" | "none" | ChannelId;
     /** Optional delivery override (E.164 for WhatsApp, chat id for Telegram). */
     to?: string;
+    /** Optional account id for multi-account channels. */
+    accountId?: string;
     /** Override the heartbeat prompt body (default: "Read HEARTBEAT.md if it exists (workspace context). Follow it strictly. Do not infer or repeat old tasks from prior chats. If nothing needs attention, reply HEARTBEAT_OK."). */
     prompt?: string;
     /** Max chars allowed after HEARTBEAT_OK before delivery (default: 30). */