Treat agent mode icons as open strings

Old clients pinned AgentModeIcon to a closed enum and rendered
undefined for unknown values, crashing the route in production
Hermes with no stack trace. Loosen the type to a plain string,
let the client skip rendering when an icon is unregistered, and
add the customModeIcons capability so the daemon downgrades
non-legacy icons to ShieldCheck for clients that pre-date the
open-string contract.

packages/server/src/shared/messages.ts

@@ -4053,6 +4053,7 @@ export const WSHelloMessageSchema = z.object({
       voice: z.boolean().optional(),
       pushNotifications: z.boolean().optional(),
       [CLIENT_CAPS.reasoningMergeEnum]: z.boolean().optional(),
+      [CLIENT_CAPS.customModeIcons]: z.boolean().optional(),
     })
     .passthrough()
     .optional(),