feat: add Z.AI chat models + 5 new providers (#3)

* feat: add Z.AI chat models (GLM-5, GLM-4.7)

Sync with librefang/librefang#409 — add the two Z.AI chat models
to the standalone model catalog.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: add 5 new providers (Stepfun, SiliconFlow, Baichuan, NVIDIA NIM, Writer)

- Stepfun (阶跃星辰): step-3.5-flash, step-3, step-2-mini, step-1o-turbo-vision
- SiliconFlow (硅基流动): inference platform, models discovered at runtime
- Baichuan (百川): Baichuan-4, Baichuan-3 Turbo 128K
- NVIDIA NIM: inference platform, models discovered at runtime
- Writer: Palmyra X5, X4, Med, Fin
- Add global aliases for stepfun, baichuan, writer

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

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

providers/baichuan.toml

@@ -0,0 +1,35 @@
+# Baichuan (百川智能) — https://platform.baichuan-ai.com
+# Models: 2
+
+[provider]
+id = "baichuan"
+display_name = "Baichuan (百川)"
+api_key_env = "BAICHUAN_API_KEY"
+base_url = "https://api.baichuan-ai.com/v1"
+key_required = true
+
+[[models]]
+id = "Baichuan4"
+display_name = "Baichuan-4"
+tier = "frontier"
+context_window = 131072
+max_output_tokens = 8192
+input_cost_per_m = 14.30
+output_cost_per_m = 14.30
+supports_tools = true
+supports_vision = false
+supports_streaming = true
+aliases = ["baichuan-4", "baichuan4"]
+
+[[models]]
+id = "Baichuan3-Turbo-128k"
+display_name = "Baichuan-3 Turbo 128K"
+tier = "smart"
+context_window = 131072
+max_output_tokens = 8192
+input_cost_per_m = 7.15
+output_cost_per_m = 7.15
+supports_tools = true
+supports_vision = false
+supports_streaming = true
+aliases = ["baichuan-3-turbo", "baichuan3-turbo"]