feat: initial model catalog with 196 models across 39 providers

Community-maintained TOML catalog for LibreFang. New models can be added
via PR without requiring a LibreFang binary release.

Includes validation script, bilingual docs, and GitHub templates.

providers/volcengine-coding.toml

@@ -0,0 +1,9 @@
+# Volcano Engine Coding Plan — https://volcengine.com
+# Models: 0 (uses same API, coding-specific endpoint)
+
+[provider]
+id = "volcengine_coding"
+display_name = "Volcano Engine Coding Plan"
+api_key_env = "VOLCENGINE_API_KEY"
+base_url = "https://ark.cn-beijing.volces.com/api/coding/v3"
+key_required = true