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>

docs/gateway/heartbeat.md

@@ -87,6 +87,7 @@ and logged; a message that is only `HEARTBEAT_OK` is dropped.
         includeReasoning: false, // default: false (deliver separate Reasoning: message when available)
         target: "last", // last | none | <channel id> (core or plugin, e.g. "bluebubbles")
         to: "+15551234567", // optional channel-specific override
+        accountId: "ops-bot", // optional multi-account channel id
         prompt: "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.",
         ackMaxChars: 300, // max chars allowed after HEARTBEAT_OK
       },
@@ -136,6 +137,35 @@ Example: two agents, only the second agent runs heartbeats.
 }
 ```
 
+### Multi account example
+
+Use `accountId` to target a specific account on multi-account channels like Telegram:
+
+```json5
+{
+  agents: {
+    list: [
+      {
+        id: "ops",
+        heartbeat: {
+          every: "1h",
+          target: "telegram",
+          to: "12345678",
+          accountId: "ops-bot",
+        },
+      },
+    ],
+  },
+  channels: {
+    telegram: {
+      accounts: {
+        "ops-bot": { botToken: "YOUR_TELEGRAM_BOT_TOKEN" },
+      },
+    },
+  },
+}
+```
+
 ### Field notes
 
 - `every`: heartbeat interval (duration string; default unit = minutes).
@@ -150,6 +180,7 @@ Example: two agents, only the second agent runs heartbeats.
   - explicit channel: `whatsapp` / `telegram` / `discord` / `googlechat` / `slack` / `msteams` / `signal` / `imessage`.
   - `none`: run the heartbeat but **do not deliver** externally.
 - `to`: optional recipient override (channel-specific id, e.g. E.164 for WhatsApp or a Telegram chat id).
+- `accountId`: optional account id for multi-account channels. When `target: "last"`, the account id applies to the resolved last channel if it supports accounts; otherwise it is ignored. If the account id does not match a configured account for the resolved channel, delivery is skipped.
 - `prompt`: overrides the default prompt body (not merged).
 - `ackMaxChars`: max chars allowed after `HEARTBEAT_OK` before delivery.