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/zai.toml

@@ -1,5 +1,5 @@
 # Z.AI — https://api.z.ai (Zhipu domain alias)
-# Models: 0 (uses same models as zhipu, different domain)
+# Models: 2
 
 [provider]
 id = "zai"
@@ -7,3 +7,29 @@ display_name = "Z.AI"
 api_key_env = "ZHIPU_API_KEY"
 base_url = "https://api.z.ai/api/paas/v4"
 key_required = true
+
+[[models]]
+id = "zai/glm-5-20250605"
+display_name = "GLM-5 (Z.AI)"
+tier = "frontier"
+context_window = 131072
+max_output_tokens = 16384
+input_cost_per_m = 1.0
+output_cost_per_m = 3.2
+supports_tools = true
+supports_vision = true
+supports_streaming = true
+aliases = ["zai-glm-5"]
+
+[[models]]
+id = "zai/glm-4.7"
+display_name = "GLM-4.7 (Z.AI)"
+tier = "smart"
+context_window = 131072
+max_output_tokens = 16384
+input_cost_per_m = 0.6
+output_cost_per_m = 2.2
+supports_tools = true
+supports_vision = true
+supports_streaming = true
+aliases = ["zai-glm-4.7"]