Feature Request: Route Heartbeats to Isolated System Session

Problem

When a heartbeat poll fires during an active conversation, the HEARTBEAT_OK response gets injected into the user's conversation thread, causing confusion.

Example:

User: Can you email me this plan?
Assistant: [sends email]
[heartbeat poll arrives]
Assistant: HEARTBEAT_OK
User: What are you talking about? Your reply doesn't match the thread at all

Proposed Solution

Heartbeat polls should:

1. Route to a dedicated "system" session that never posts to active user conversations
2. OR be suppressed entirely when the main session has recent (< 30 min) user activity
3. OR heartbeat responses should be marked as "silent" and not rendered in the conversation thread

Current Workaround

Agents can detect mid-conversation state and skip heartbeat responses, but this is fragile and relies on agent-side logic.

Impact

User confusion, broken conversation flow, loss of trust when the assistant appears to be talking nonsense.