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/client/daemon-client.ts

@@ -4264,6 +4264,7 @@ export class DaemonClient {
           protocolVersion: 1,
           capabilities: {
             [CLIENT_CAPS.reasoningMergeEnum]: true,
+            [CLIENT_CAPS.customModeIcons]: true,
           },
           ...(this.config.appVersion ? { appVersion: this.config.appVersion } : {}),
         }),