feat(discord): add set-presence action for bot activity and status

Bridge the agent tools layer to the Discord gateway WebSocket via a new
gateway registry, allowing agents to set the bot's activity and online
status. Supports playing, streaming, listening, watching, custom, and
competing activity types. Custom type uses activityState as the sidebar
text; other types show activityName in the sidebar and activityState in
the flyout. Opt-in via channels.discord.actions.presence (default false).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/discord/monitor/gateway-registry.ts

@@ -0,0 +1,33 @@
+import type { GatewayPlugin } from "@buape/carbon/gateway";
+
+/**
+ * Module-level registry of active Discord GatewayPlugin instances.
+ * Bridges the gap between agent tool handlers (which only have REST access)
+ * and the gateway WebSocket (needed for operations like updatePresence).
+ * Follows the same pattern as presence-cache.ts.
+ */
+const gatewayRegistry = new Map<string, GatewayPlugin>();
+
+function resolveAccountKey(accountId?: string): string {
+  return accountId ?? "default";
+}
+
+/** Register a GatewayPlugin instance for an account. */
+export function registerGateway(accountId: string | undefined, gateway: GatewayPlugin): void {
+  gatewayRegistry.set(resolveAccountKey(accountId), gateway);
+}
+
+/** Unregister a GatewayPlugin instance for an account. */
+export function unregisterGateway(accountId?: string): void {
+  gatewayRegistry.delete(resolveAccountKey(accountId));
+}
+
+/** Get the GatewayPlugin for an account. Returns undefined if not registered. */
+export function getGateway(accountId?: string): GatewayPlugin | undefined {
+  return gatewayRegistry.get(resolveAccountKey(accountId));
+}
+
+/** Clear all registered gateways (for testing). */
+export function clearGateways(): void {
+  gatewayRegistry.clear();
+}