Fixes #20196

Summary

• Guard enqueueSystemEvent + requestHeartbeatNow in dispatchAgentHook behind !result.delivered, preventing duplicate messages when hook agent turns deliver via the announce flow
• Mirrors the same fix applied to executeJobCore in the cron service path (ea95e88, [Bug]: cron isolated agentTurn with delivery.mode=announce posts duplicate messages with different content (extra agent run triggered) #15692)

Problem

When /hooks/agent is called with deliver: true and a specific agentId + to target, the isolated agent turn delivers the response correctly via the subagent announce flow. However, dispatchAgentHook then unconditionally posts a summary to the main agent session via enqueueSystemEvent and wakes the heartbeat via requestHeartbeatNow. This causes the main agent to wake up, process the system event, and generate a second response — resulting in duplicate messages delivered to the target channel (or to the main agent's last active chat).

Fix

The RunCronAgentTurnResult.delivered flag already exists and is documented:

true when the isolated run already delivered its output to the target channel. Callers should skip posting a summary to the main session to avoid duplicate messages.

The cron service path (timer.ts) already honors this flag since ea95e88. This PR applies the same guard to the hooks handler.

Test plan

• Trigger a hook with deliver: true targeting a non-default agent → verify response arrives only in the target chat
• Trigger a hook with deliver: false → verify summary still posts to main session and heartbeat wakes
• Trigger a hook that fails → verify error summary still posts to main session

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