Added a Right-Hand Sidebar (RHS) panel to the Mattermost plugin that
shows live agent activity in a dedicated, always-visible panel.
- New RHSPanel component with SessionCard views per active session
- registerAppBarComponent adds 'Agent Status' icon to toolbar
- Subscribes to WebSocket updates via global listener
- Shows active sessions with live elapsed time, tool calls, token count
- Shows recent completed sessions below active ones
- Responsive CSS matching Mattermost design system
The RHS panel solves the scroll-out-of-view problem: the status
dashboard stays visible regardless of chat scroll position.
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