feat: add GitHub Copilot model provider with OAuth device flow

Add a new 'github' model provider backed by the Copilot API
(api.githubcopilot.com), with OAuth device flow for authentication.

- New src/auth/github.ts: device flow login, token storage at
  ~/.config/flynn/auth.json with 0600 permissions
- New src/models/github.ts: OpenAI-compatible client with streaming,
  tool calling, and Copilot-specific headers
- Add 'github' to provider enum in config schema
- Register provider in daemon factory and TUI client factory
- Refactor TUI to use provider-agnostic client factory (was hardcoded
  to AnthropicClient for all tiers)
- Add /login command to TUI for interactive OAuth authorization
- Add Copilot model cost tracking entries

src/auth/index.ts

@@ -0,0 +1,9 @@
+export {
+  requestDeviceCode,
+  pollForToken,
+  loadStoredToken,
+  storeToken,
+  getGitHubToken,
+  loginGitHub,
+  type DeviceCodeResponse,
+} from './github.js';