feat: add GitHub Copilot provider

Copilot device login + onboarding option; model list auth detection.

src/providers/github-copilot-models.ts

@@ -0,0 +1,41 @@
+import type { ModelDefinitionConfig } from "../config/types.js";
+
+const DEFAULT_CONTEXT_WINDOW = 128_000;
+const DEFAULT_MAX_TOKENS = 8192;
+
+// Copilot model ids vary by plan/org and can change.
+// We keep this list intentionally broad; if a model isn't available Copilot will
+// return an error and users can remove it from their config.
+const DEFAULT_MODEL_IDS = [
+  "gpt-4o",
+  "gpt-4.1",
+  "gpt-4.1-mini",
+  "gpt-4.1-nano",
+  "o1",
+  "o1-mini",
+  "o3-mini",
+] as const;
+
+export function getDefaultCopilotModelIds(): string[] {
+  return [...DEFAULT_MODEL_IDS];
+}
+
+export function buildCopilotModelDefinition(
+  modelId: string,
+): ModelDefinitionConfig {
+  const id = modelId.trim();
+  if (!id) throw new Error("Model id required");
+  return {
+    id,
+    name: id,
+    // pi-coding-agent's registry schema doesn't know about a "github-copilot" API.
+    // We use OpenAI-compatible responses API, while keeping the provider id as
+    // "github-copilot" (pi-ai uses that to attach Copilot-specific headers).
+    api: "openai-responses",
+    reasoning: false,
+    input: ["text", "image"],
+    cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+    contextWindow: DEFAULT_CONTEXT_WINDOW,
+    maxTokens: DEFAULT_MAX_TOKENS,
+  };
+}