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/models/index.ts

@@ -2,6 +2,7 @@ export { AnthropicClient, type AnthropicClientConfig } from './anthropic.js';
 export { OpenAIClient, type OpenAIClientConfig } from './openai.js';
 export { GeminiClient, type GeminiClientConfig } from './gemini.js';
 export { BedrockClient, type BedrockClientConfig } from './bedrock.js';
+export { GitHubModelsClient, type GitHubModelsClientConfig } from './github.js';
 export { OllamaClient, type OllamaClientConfig } from './local/index.js';
 export { LlamaCppClient, type LlamaCppClientConfig } from './local/index.js';
 export { ModelRouter, type ModelRouterConfig, type ModelTier } from './router.js';