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

@@ -14,6 +14,7 @@ export const defaultModelPerProvider: Record<KnownProvider, string> = {
 	anthropic: "claude-sonnet-4-5",
 	openai: "gpt-5.1-codex",
 	google: "gemini-2.5-pro",
+	"github-copilot": "gpt-4o",
 	openrouter: "openai/gpt-5.1-codex",
 	xai: "grok-4-fast-non-reasoning",
 	groq: "openai/gpt-oss-120b",