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,

frontend/src/pages/AssistantChatPage.tsx

@@ -82,7 +82,6 @@ export default function AssistantChatPage() {
   const [magicHandoff, setMagicHandoff] = useState<HandoffResponse | null>(null)
   const [overlayHandoff, setOverlayHandoff] = useState<HandoffResponse | null>(null)
   const [overlayLoading, setOverlayLoading] = useState(false)
-  const [claiming, setClaiming] = useState(false)
   const [activeOptionKey, setActiveOptionKey] = useState<'continue' | 'ai' | 'own' | null>(null)
   // Codex correction (locked design): once the magic-moment dissolves, the
   // AI's `suggested_steps[]` should still be reachable as chips below the
@@ -331,52 +330,6 @@ export default function AssistantChatPage() {
     return () => { cancelled = true }
   }, [isPickup, urlSessionId, magicState, setSearchParams])
 
-  const handleStartHere = useCallback(async () => {
-    if (!urlSessionId || !magicHandoff) return
-    setClaiming(true)
-    try {
-      await handoffsApi.claimHandoff(urlSessionId, magicHandoff.id)
-      // Drop ?pickup=true and dismiss the magic-moment. The session-load
-      // effect above will then fire because magicState !== 'loading'/'visible'
-      // and selectChat will populate the chat surface — the senior is now
-      // escalated_to_id, so GET succeeds and the conversation_messages render
-      // as chat history.
-      setSearchParams({})
-      setMagicState('dismissed')
-      // Refresh the sidebar list. Pre-claim the session was invisible to
-      // listSessions because escalated_to_id was null (junior didn't
-      // specify a target on /escalate). Post-claim claim_session sets
-      // escalated_to_id = teamadmin.id, so the session is now in scope.
-      // Without this re-fetch the senior lands on a session with no
-      // sidebar entry — looks like the page navigated to a different
-      // session.
-      void loadChats()
-    } catch (e: unknown) {
-      // Race-condition path (locked design): the loser of the simultaneous
-      // Pick Up gets a 409 with structured detail so we can name the
-      // winner and approximate "how long ago." Drop the magic-moment
-      // (the session is no longer theirs to claim) and let them go back
-      // to the queue.
-      if (axios.isAxiosError(e) && e.response?.status === 409) {
-        const detail = e.response.data?.detail as
-          | { error?: string; claimed_by_name?: string; claimed_at?: string }
-          | undefined
-        if (detail?.error === 'already_claimed') {
-          const name = detail.claimed_by_name || 'another engineer'
-          const when = detail.claimed_at ? timeAgo(detail.claimed_at) : 'just now'
-          toast.info(`Already claimed by ${name} ${when}.`)
-          setSearchParams({})
-          setMagicState('dismissed')
-          return
-        }
-      }
-      const message = e instanceof Error ? e.message : 'Failed to pick up session'
-      toast.error(message)
-    } finally {
-      setClaiming(false)
-    }
-  }, [urlSessionId, magicHandoff, setSearchParams])
-
   const handleContinue = useCallback(async () => {
     if (!urlSessionId || !magicHandoff) return
     setActiveOptionKey('continue')