feat: update Volcengine with Doubao Seed 2.0 and 1.8 models (#4)

- Add Doubao Seed 2.0 Pro (frontier, 256K context, 128K output)
- Add Doubao Seed 2.0 Code
- Add Doubao Seed 1.8 (multimodal agent model with vision)
- Add Doubao Seed 1.6 Vision
- Update existing 2.0 Lite/Mini with correct specs (256K/128K, vision)
- Update global alias "doubao" to point to 2.0 Pro

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

aliases.toml

@@ -77,6 +77,8 @@
 "minimax-highspeed" = "MiniMax-M2.5-highspeed"
 "minimax-m2.1" = "MiniMax-M2.1"
 "codegeex" = "codegeex-4"
+"doubao" = "doubao-seed-2-0-pro-260215"
+"doubao-seed" = "doubao-seed-2-0-pro-260215"
 "stepfun" = "step-3.5-flash"
 "baichuan" = "Baichuan4"
 "baichuan-4" = "Baichuan4"