feat: add VK channel support (#2276)

* feat: add VK channel support

- Add VK channel implementation using vksdk
- Support text messages and media attachments
- Implement Long Poll API for real-time messaging
- Add group chat support with trigger prefixes
- Add user whitelist (allow_from) configuration
- Add VK channel documentation

Files:
- pkg/channels/vk/: VK channel implementation
- pkg/config/config.go: Add VKConfig structure
- pkg/channels/manager.go: Register VK channel
- pkg/gateway/gateway.go: Import VK channel package
- docs/channels/vk/: Usage documentation

* test: add unit tests for VK channel

- Test channel initialization with various configurations
- Test allow_from whitelist functionality
- Test group trigger configuration
- Test max message length (4000 chars)
- Test message splitting logic
- Test attachment processing

All tests passing ✓

* fix: resolve linting issues in VK channel

- Format VKConfig struct tags to comply with golines
- Remove unused mu sync.Mutex field
- Remove unused stripPrefix method

All tests passing ✓

* style: format VKConfig with golines

- Align struct tags to match project style
- Match formatting with other channel configs (Telegram, etc.)
- Fix golines linting error

* style: fix struct tag formatting in config.go

* docs: update VK channel docs to use secure token storage

* feat(vk): add voice capabilities support

- Implement VoiceCapabilities() method for VK channel
- Add audio_message attachment handling in processAttachments
- Add comprehensive tests for voice capabilities
- Support both ASR (speech-to-text) and TTS (text-to-speech)

* docs: add VK channel to documentation and update voice support

- Add VK channel to README.md and README.zh.md channel lists
- Update VK channel documentation with voice message support
- Document ASR and TTS capabilities for VK channel
- Add voice transcription configuration reference

README.md

@@ -454,7 +454,7 @@ For full provider configuration details, see [Providers & Models](docs/providers
 
 ## 💬 Channels (Chat Apps)
 
-Talk to your PicoClaw through 17+ messaging platforms:
+Talk to your PicoClaw through 18+ messaging platforms:
 
 | Channel | Setup | Protocol | Docs |
 |---------|-------|----------|------|
@@ -469,6 +469,7 @@ Talk to your PicoClaw through 17+ messaging platforms:
 | **Feishu / Lark** | Medium (App ID + Secret) | WebSocket/SDK | [Guide](docs/channels/feishu/README.md) |
 | **LINE** | Medium (credentials + webhook) | Webhook | [Guide](docs/channels/line/README.md) |
 | **WeCom** | Easy (QR login or manual) | WebSocket | [Guide](docs/channels/wecom/README.md) |
+| **VK** | Easy (group token) | Long Poll | [Guide](docs/channels/vk/README.md) |
 | **IRC** | Medium (server + nick) | IRC protocol | [Guide](docs/chat-apps.md#irc) |
 | **OneBot** | Medium (WebSocket URL) | OneBot v11 | [Guide](docs/channels/onebot/README.md) |
 | **MaixCam** | Easy (enable) | TCP socket | [Guide](docs/channels/maixcam/README.md) |