feat: RHS panel — persistent Agent Status sidebar

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.

plugin/webapp/src/styles/live_status.css

@@ -195,3 +195,113 @@
 .ls-terminal::-webkit-scrollbar-thumb:hover {
   background: var(--center-channel-color-32, rgba(0, 0, 0, 0.32));
 }
+
+/* ========= RHS Panel Styles ========= */
+
+.ls-rhs-panel {
+  display: flex;
+  flex-direction: column;
+  height: 100%;
+  overflow-y: auto;
+  background: var(--center-channel-bg, #fff);
+}
+
+.ls-rhs-summary {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 12px 16px;
+  border-bottom: 1px solid var(--center-channel-color-08, rgba(0, 0, 0, 0.08));
+  font-size: 13px;
+  font-weight: 600;
+  color: var(--center-channel-color, #3d3c40);
+}
+
+.ls-rhs-count {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+}
+
+.ls-rhs-empty {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  padding: 48px 24px;
+  text-align: center;
+}
+
+.ls-rhs-empty-icon {
+  font-size: 32px;
+  margin-bottom: 12px;
+}
+
+.ls-rhs-empty-text {
+  color: var(--center-channel-color-48, rgba(0, 0, 0, 0.48));
+  font-size: 13px;
+  line-height: 1.5;
+}
+
+.ls-rhs-card {
+  margin: 8px 12px;
+  border: 1px solid var(--center-channel-color-08, rgba(0, 0, 0, 0.08));
+  border-radius: 6px;
+  overflow: hidden;
+}
+
+.ls-rhs-card.ls-status-active {
+  border-color: rgba(76, 175, 80, 0.3);
+}
+
+.ls-rhs-card-header {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 8px 10px;
+  background: var(--center-channel-color-04, rgba(0, 0, 0, 0.04));
+  border-bottom: 1px solid var(--center-channel-color-08, rgba(0, 0, 0, 0.08));
+}
+
+.ls-rhs-card .ls-terminal {
+  max-height: 250px;
+  font-size: 11px;
+  padding: 6px 10px;
+}
+
+.ls-rhs-card-footer {
+  padding: 4px 10px 6px;
+  font-size: 11px;
+  color: var(--center-channel-color-48, rgba(0, 0, 0, 0.48));
+  font-family: 'SFMono-Regular', Consolas, monospace;
+  border-top: 1px solid var(--center-channel-color-08, rgba(0, 0, 0, 0.08));
+}
+
+.ls-rhs-children {
+  padding: 4px 10px;
+  border-top: 1px solid var(--center-channel-color-08, rgba(0, 0, 0, 0.08));
+}
+
+.ls-rhs-child {
+  padding: 4px 0;
+}
+
+.ls-rhs-child-header {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  font-size: 11px;
+}
+
+.ls-rhs-section {
+  margin-top: 4px;
+}
+
+.ls-rhs-section-title {
+  padding: 8px 16px 4px;
+  font-size: 11px;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.5px;
+  color: var(--center-channel-color-48, rgba(0, 0, 0, 0.48));
+}