feat: Agent Health page with progressive filtering and GroupCard component

Add URL-driven Agent Health page (/agents, /agents/:appId,
/agents/:appId/:instanceId) that progressively narrows from all
applications to a single instance with trend charts. Create
generic GroupCard composite for grouping instances by application.
Expand mock data to 8 instances across 4 apps with varied states.
Split sidebar Agents header into navigable link + collapse chevron.
Update agent tree paths to /agents/:appId/:instanceId. Add EventFeed
with lifecycle events. Change SidebarAgent.tps from string to number.

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

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

@@ -215,6 +215,40 @@
   justify-content: center;
 }
 
+/* Section header as link (for Agents nav) */
+.treeSectionLink {
+  font-size: 10px;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 1px;
+  color: var(--sidebar-muted);
+  text-decoration: none;
+  flex: 1;
+  transition: color 0.12s;
+}
+
+.treeSectionLink:hover {
+  color: var(--amber-light);
+}
+
+.treeSectionChevronBtn {
+  background: none;
+  border: none;
+  padding: 2px 4px;
+  margin: 0;
+  color: var(--sidebar-muted);
+  font-size: 9px;
+  cursor: pointer;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  transition: color 0.12s;
+}
+
+.treeSectionChevronBtn:hover {
+  color: var(--sidebar-text);
+}
+
 .tree {
   list-style: none;
   margin: 0;