Zero-idle sessions: SDK never emits session.idle event

[PureWeen]

Problem

Some copilot CLI sessions never emit a session.idle event after the turn completes. This means CompleteResponse never fires, leaving IsProcessing=true until the watchdog kills it.

Current workaround

• FlushCurrentResponse on AssistantTurnEndEvent persists content so it's not lost
• 5-second heartbeat timer in Dashboard catches missed UI renders
• Watchdog eventually clears stuck processing state

Root cause

Unknown — appears to be a copilot CLI/SDK bug in headless mode. The SDK emits assistant.turn_end but never follows with session.idle.

Desired fix

Investigate why session.idle is missing and fix at the SDK level. Once fixed, the heartbeat interval can be relaxed from 5s to 15-30s (keep as defense-in-depth).

References

• PR Session resilience: heartbeat timer, permission auto-recovery, activity-aware watchdog #285 (heartbeat + liveness signal)