feat(ai): provider factories, per-provider catalogs, createProvider (phase 3)

Auth helpers in src/auth/helpers.ts: envApiKeyAuth() (stored key wins,
then env vars in order, with secret-prompt login) and lazyOAuth()
(flow loads on first use through bundler-opaque dynamic imports in
utils/oauth/load.ts; the OAuthAuth flow exports land in phase 4).
There is no OAuth factory toggle: providers that support OAuth always
attach it, advertising costs nothing until login/refresh runs.

createProvider() in models.ts builds providers from parts: single API
implementation or a map dispatched on model.api (mixed-API providers
like opencode and github-copilot); unknown api yields a stream error.

generate-models.ts now emits one providers/<id>.models.ts catalog per
provider (35 files, biome-excluded like models.generated.ts) and
models.generated.ts becomes a generated aggregator, so importing one
provider factory pulls one catalog. Typed getModel globals unchanged.

One factory per built-in provider under src/providers/: envApiKeyAuth
for standard providers, OAuth for anthropic/openai-codex/github-copilot,
ambient ApiKeyAuth for amazon-bedrock (AWS env/profile/IAM) and
google-vertex (explicit key or ADC+project+location).

providers/all.ts: builtinProviders(), builtinModels(), getBuiltin*
re-exports. fauxProvider() factory returns a real Provider for tests;
legacy registerFauxProvider() unchanged.

packages/ai/src/providers/github-copilot.ts

@@ -0,0 +1,25 @@
+import { anthropicMessagesApi } from "../api/anthropic-messages.lazy.ts";
+import { openAICompletionsApi } from "../api/openai-completions.lazy.ts";
+import { openAIResponsesApi } from "../api/openai-responses.lazy.ts";
+import { envApiKeyAuth, lazyOAuth } from "../auth/helpers.ts";
+import { createProvider, type Provider } from "../models.ts";
+import { loadGitHubCopilotOAuth } from "../utils/oauth/load.ts";
+import { GITHUB_COPILOT_MODELS } from "./github-copilot.models.ts";
+
+export function githubCopilotProvider(): Provider<"anthropic-messages" | "openai-completions" | "openai-responses"> {
+	return createProvider({
+		id: "github-copilot",
+		name: "GitHub Copilot",
+		baseUrl: "https://api.individual.githubcopilot.com",
+		auth: {
+			apiKey: envApiKeyAuth("GitHub Copilot token", ["COPILOT_GITHUB_TOKEN"]),
+			oauth: lazyOAuth({ name: "GitHub Copilot", load: loadGitHubCopilotOAuth }),
+		},
+		models: Object.values(GITHUB_COPILOT_MODELS),
+		api: {
+			"anthropic-messages": anthropicMessagesApi(),
+			"openai-completions": openAICompletionsApi(),
+			"openai-responses": openAIResponsesApi(),
+		},
+	});
+}