fix: session.idle with stale background-task payload no longer blocks IsProcessing

[PureWeen]

Summary

• What broke: Race in IDLE-DEFER — backgroundTasksChanged shells=0 fires (PolyPilot confirms all shells done), then session.idle arrives with shells=2 (stale CLI snapshot generated before completions landed). PolyPilot started a fresh 60-min zombie clock from the stale payload, keeping IsProcessing=true indefinitely.
• What the fix does: Capture preIdleFingerprint / preIdleTicks before calling RefreshDeferredBackgroundTaskTracking. If they were null/0 (meaning backgroundTasksChanged already confirmed empty), the session.idle payload is stale — skip the defer entirely. PolyPilot's real-time backgroundTasksChanged tracking is the ground truth over the CLI's snapshot.
• Which invariant it restores: INV-O3 / IDLE-DEFER safety — a session stuck with IsProcessing=true and no active background tasks must eventually complete. The zombie timeout was being bypassed by the timer reset.

Observed failure

PROMPT session — last console.log entries:

17:12:16 [BG-TASKS] 'PROMPT' background tasks changed (agents=0, shells=0, ...)
17:12:21 [BG-TASKS] 'PROMPT' background tasks changed (agents=0, shells=0, ...)
17:12:27 [IDLE-DIAG] 'PROMPT' session.idle payload: backgroundTasks={agents=0, shells=2, null=False}
17:12:27 [IDLE-DEFER] 'PROMPT' session.idle received with active background tasks — deferring completion (IsProcessing=True, ...)
17:15:38 [WATCHDOG] 'PROMPT' Case B deferred — events.jsonl modified since turn start (deferral=1/40, freshness=1800s)

PolyPilot's own tracking: shells=0. CLI idle payload: shells=2. Session stuck.

Test plan

• SessionIdle_StalePayload_NotDeferredWhenBgTasksAlreadyConfirmedEmpty — structural test verifying the staleness guard is present and wired correctly
• Existing BackgroundTasksIdleTests pass (14/16; 2 pre-existing unrelated failures on SessionBackgroundTasksChangedEvent string search)
• Verify PROMPT session exits IsProcessing after next PolyPilot relaunch with this fix

[PureWeen]

Changes folded into PR #528.