feat: initial impl

- add GitHub Copilot model discovery (env token fallback, headers,
compat) plus fallback list and quoted provider keys in generated map
- surface Copilot provider end-to-end (KnownProvider/default, env+OAuth
token refresh/save, enterprise base URL swap, available only when
creds/env exist)
- tweak interactive OAuth UI to render instruction text and prompt
placeholders

gpt-5.2-high took about 35 minutes. It had a lot of trouble with `npm
check`  and went off on a "let's adjust every tsconfig" side quest.
Device code flow works, but the ai/scripts/generate-models.ts impl is
wrong as models from months ago are missing and only those deprecated
are accessible in the /models picker.

packages/ai/src/stream.ts

@@ -31,6 +31,10 @@ export function getApiKey(provider: any): string | undefined {
 	if (key) return key;
 
 	// Fall back to environment variables
+	if (provider === "github-copilot") {
+		return process.env.COPILOT_GITHUB_TOKEN || process.env.GH_TOKEN || process.env.GITHUB_TOKEN;
+	}
+
 	const envMap: Record<string, string> = {
 		openai: "OPENAI_API_KEY",
 		anthropic: "ANTHROPIC_API_KEY",