feat(opencode): support custom agents and slash commands

Remove the hardcoded mode whitelist so user-defined agents (from
opencode.json or global config) appear in the mode picker. Add a
listProviderModes API (client → session → provider) so the draft
agent form fetches dynamic modes instead of relying on the static
manifest. Fix slash command dispatch to accept optional arguments.

Closes #185

packages/server/src/client/daemon-client.ts

@@ -38,6 +38,7 @@ import type {
   ArchiveWorkspaceResponseMessage,
   ListCommandsResponse,
   ListProviderModelsResponseMessage,
+  ListProviderModesResponseMessage,
   ListAvailableProvidersResponse,
   ListTerminalsResponse,
   CreateTerminalResponse,
@@ -216,6 +217,7 @@ type CreatePaseoWorktreePayload = Extract<
 type FileExplorerPayload = FileExplorerResponse["payload"];
 type FileDownloadTokenPayload = FileDownloadTokenResponse["payload"];
 type ListProviderModelsPayload = ListProviderModelsResponseMessage["payload"];
+type ListProviderModesPayload = ListProviderModesResponseMessage["payload"];
 type ListAvailableProvidersPayload = ListAvailableProvidersResponse["payload"];
 type ListCommandsPayload = ListCommandsResponse["payload"];
 type ListCommandsDraftConfig = Pick<
@@ -2474,6 +2476,22 @@ export class DaemonClient {
     });
   }
 
+  async listProviderModes(
+    provider: AgentProvider,
+    options?: { cwd?: string; requestId?: string },
+  ): Promise<ListProviderModesPayload> {
+    return this.sendCorrelatedSessionRequest({
+      requestId: options?.requestId,
+      message: {
+        type: "list_provider_modes_request",
+        provider,
+        cwd: options?.cwd,
+      },
+      responseType: "list_provider_modes_response",
+      timeout: 45000,
+    });
+  }
+
   async listAvailableProviders(options?: {
     requestId?: string;
   }): Promise<ListAvailableProvidersPayload> {