fix(agent-manager): resolve sessionsLoaded race condition with webview mount (#539)

The loading skeleton gates (worktreesLoaded && sessionsLoaded) could
permanently block the Agent Manager UI. initializeState() called
refreshSessions() which sent sessionsLoaded before the webview's
onMount handlers registered, so the signal was lost. Since sessions
were already populated, the MessageList backup effect never triggered
a retry.

Add refreshSessions() to the requestState handler which fires from
the webview's onMount, guaranteeing all message listeners are ready.