feat: Add MCP Servers management UI

Full MCP server lifecycle: add (stdio + HTTP), edit, remove, test,
enable/disable. YAML config patching for args, env, headers, tool
filters, timeouts. OAuth token detection + deletion. Preset picker
for common MCP servers. Gateway restart banner after config changes.

New sidebar section "MCP Servers" under Manage.

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

scarf/scarf/Navigation/AppCoordinator.swift

@@ -10,6 +10,7 @@ enum SidebarSection: String, CaseIterable, Identifiable {
     case memory = "Memory"
     case skills = "Skills"
     case tools = "Tools"
+    case mcpServers = "MCP Servers"
     case gateway = "Gateway"
     case cron = "Cron"
     case health = "Health"
@@ -29,6 +30,7 @@ enum SidebarSection: String, CaseIterable, Identifiable {
         case .memory: return "brain"
         case .skills: return "lightbulb"
         case .tools: return "wrench.and.screwdriver"
+        case .mcpServers: return "puzzlepiece.extension"
         case .gateway: return "antenna.radiowaves.left.and.right"
         case .cron: return "clock.arrow.2.circlepath"
         case .health: return "stethoscope"