feat(channels): add multimodal support with image handling

- Add model configuration option for channel-specific model selection
- Support base64-encoded images in prompts via AcpBridge
- Add media utilities for WeChat/Weixin channel
- Update settings schema for model configuration

Enables channels to process images and use custom models.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/cli/src/commands/channel/start.ts

@@ -104,6 +104,7 @@ export const startCommand: CommandModule<object, { name: string }> = {
       cwd: (rawConfig['cwd'] as string) || process.cwd(),
       approvalMode: rawConfig['approvalMode'] as string | undefined,
       instructions: rawConfig['instructions'] as string | undefined,
+      model: rawConfig['model'] as string | undefined,
       groupPolicy:
         (rawConfig['groupPolicy'] as ChannelConfig['groupPolicy']) ||
         'disabled',
@@ -120,7 +121,11 @@ export const startCommand: CommandModule<object, { name: string }> = {
     writeStdoutLine(`[Channel] CLI entry: ${cliEntryPath}`);
     writeStdoutLine(`[Channel] Starting "${name}" (type=${config.type})...`);
 
-    const bridge = new AcpBridge({ cliEntryPath, cwd: config.cwd });
+    const bridge = new AcpBridge({
+      cliEntryPath,
+      cwd: config.cwd,
+      model: config.model,
+    });
     await bridge.start();
 
     let channel: TelegramChannel | WeixinChannel;