feat: implement tool instances backend and session navigation

Backend:
- Create ToolInstance model with status tracking
- Add Alembic migration for tool_instances table
- Create Docker service for compose template rendering and container execution
- Add CRUD API endpoints for tool instances
- Add lifecycle endpoints (start/stop/restart)
- Add user sessions endpoint for navigation
- Register routers in main.py

Frontend:
- Create SessionsProvider with React context
- Create sessions API client
- Update AppShell with sessions section in navigation
- Add session status indicators and polling
- Add CSS for session navigation

Quality gates: typecheck ✓, lint ✓, build ✓

apps/web/src/styles.css

@@ -2217,3 +2217,46 @@ a.nav-item,
     align-items: center;
   }
 }
+
+/* Session Navigation */
+.session-item {
+  position: relative;
+  padding-left: var(--space-6);
+}
+
+.session-status {
+  position: absolute;
+  left: var(--space-2);
+  top: 50%;
+  transform: translateY(-50%);
+  width: 8px;
+  height: 8px;
+  border-radius: 50%;
+  background: var(--muted);
+}
+
+.session-status.running {
+  background: var(--success);
+}
+
+.session-name {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  max-width: 140px;
+}
+
+.nav-divider {
+  height: 1px;
+  background: var(--border);
+  margin: var(--space-2) var(--space-3);
+}
+
+.nav-section-title {
+  padding: var(--space-2) var(--space-4);
+  font-size: var(--text-xs);
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+  color: var(--muted);
+}