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.

.github/pull_request_template.md

@@ -0,0 +1,14 @@
+## Model Changes
+
+### Added/Updated Models
+- [ ] Model ID:
+- [ ] Provider:
+- [ ] Pricing verified from official source:
+
+### Checklist
+- [ ] `python scripts/validate.py` passes
+- [ ] No duplicate model IDs
+- [ ] Pricing is in USD per million tokens
+- [ ] Tier is one of: `frontier`, `smart`, `balanced`, `fast`, `local`
+- [ ] `context_window` and `max_output_tokens` are positive integers
+- [ ] Boolean fields (`supports_tools`, `supports_vision`, `supports_streaming`) are correct