fix(escalations): atomic claim + self-claim rejection + queue exclusion

Codex review pass on the escalation wedge. Reworks claim_session from
read-then-write to a conditional UPDATE so two seniors racing can't both
win, blocks the original engineer from claiming their own handoff, and
filters self-escalated sessions out of the dashboard escalation queue.
Also preassigns the handoff UUID before flush so the compatibility
escalation_package payload carries it. Removes legacy frontend pickup
state (claiming, handleStartHere) that broke tsc --noEmit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/flowpilot/HandoffContextScreen.tsx

@@ -90,7 +90,6 @@ export function HandoffContextScreen({
   onContinue,
   onAIAnalysis,
   onOwnThing,
-  onStartHere,
   onDismiss,
   dismissible = false,
   isProcessing = false,