fix(ai,coding-agent): drop models retired upstream

models.dev deprecated several Baseten, GitHub Copilot, and Together models,
which broke type checks and the default-model test.

- Default GitHub Copilot model: gpt-5.4 -> gpt-6.1-sol
- Move Copilot live tests to gpt-6-luna/gpt-6-sol
- Remove tests and generator entries for retired models, including the
  Together reasoning-only model list

packages/coding-agent/CHANGELOG.md

@@ -9,6 +9,7 @@
 ### Changed
 
 - `generateSummary()` and `generateSummaryWithUsage()` now ignore their `reserveTokens` parameter. Summary requests use the model's output limit instead.
+- Changed the default GitHub Copilot model from `gpt-5.4`, which Copilot retired, to `gpt-6.1-sol`.
 
 ### Fixed
 

packages/coding-agent/src/core/model-resolver.ts

@@ -29,7 +29,7 @@ export const defaultModelPerProvider: Partial<Record<KnownProvider, string>> = {
 	deepseek: "deepseek-v4-pro",
 	google: "gemini-3.1-pro-preview",
 	"google-vertex": "gemini-3.1-pro-preview",
-	"github-copilot": "gpt-5.4",
+	"github-copilot": "gpt-6.1-sol",
 	openrouter: "moonshotai/kimi-k2.6",
 	"vercel-ai-gateway": "zai/glm-5.1",
 	xai: "grok-4.7",