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/writer.toml

@@ -0,0 +1,61 @@
+# Writer — https://writer.com
+# Models: 4
+
+[provider]
+id = "writer"
+display_name = "Writer"
+api_key_env = "WRITER_API_KEY"
+base_url = "https://api.writer.com/v1"
+key_required = true
+
+[[models]]
+id = "palmyra-x5"
+display_name = "Palmyra X5"
+tier = "frontier"
+context_window = 1048576
+max_output_tokens = 8192
+input_cost_per_m = 0.60
+output_cost_per_m = 6.00
+supports_tools = true
+supports_vision = true
+supports_streaming = true
+aliases = ["palmyra-x5", "writer-x5"]
+
+[[models]]
+id = "palmyra-x4"
+display_name = "Palmyra X4"
+tier = "smart"
+context_window = 131072
+max_output_tokens = 4096
+input_cost_per_m = 2.50
+output_cost_per_m = 10.00
+supports_tools = true
+supports_vision = false
+supports_streaming = true
+aliases = ["palmyra-x4", "writer-x4"]
+
+[[models]]
+id = "palmyra-med"
+display_name = "Palmyra Med"
+tier = "smart"
+context_window = 32768
+max_output_tokens = 4096
+input_cost_per_m = 5.00
+output_cost_per_m = 12.00
+supports_tools = false
+supports_vision = false
+supports_streaming = true
+aliases = ["palmyra-med", "writer-med"]
+
+[[models]]
+id = "palmyra-fin"
+display_name = "Palmyra Fin"
+tier = "smart"
+context_window = 131072
+max_output_tokens = 4096
+input_cost_per_m = 5.00
+output_cost_per_m = 12.00
+supports_tools = false
+supports_vision = false
+supports_streaming = true
+aliases = ["palmyra-fin", "writer-fin"]