feat: enhance EventFeed timeline and Agent Health page styling

- EventFeed: 28px icon circles with severity colors, stacked message/timestamp,
  search input with clear button, ReactNode message support with searchText field
- Agent Health: timeline wrapped in card panel, instance listing as proper table,
  colored Badge pills for live counts, removed shift pill
- Input primitive: onClear prop with × button for all search fields
- Sidebar: Agents section collapsible like Applications, collapse state persisted
  to localStorage
- FilterBar/Sidebar: clear buttons on all search inputs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/design-system/layout/Sidebar/Sidebar.module.css

@@ -54,7 +54,7 @@
   background: rgba(255, 255, 255, 0.06);
   border: 1px solid rgba(255, 255, 255, 0.08);
   border-radius: var(--radius-sm);
-  padding: 6px 10px 6px 28px;
+  padding: 6px 26px 6px 28px;
   color: var(--sidebar-text);
   font-family: var(--font-body);
   font-size: 12px;
@@ -80,6 +80,32 @@
   align-items: center;
 }
 
+.searchClear {
+  position: absolute;
+  right: 4px;
+  top: 50%;
+  transform: translateY(-50%);
+  width: 20px;
+  height: 20px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  border: none;
+  background: none;
+  color: var(--sidebar-muted);
+  font-size: 14px;
+  cursor: pointer;
+  border-radius: var(--radius-sm);
+  padding: 0;
+  line-height: 1;
+  transition: color 0.1s, background 0.1s;
+}
+
+.searchClear:hover {
+  color: var(--sidebar-text);
+  background: rgba(255, 255, 255, 0.08);
+}
+
 /* Scrollable nav area */
 .navArea {
   flex: 1;