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/costs.ts

@@ -14,6 +14,11 @@ export const MODEL_COSTS_PER_MILLION: Record<string, { input: number; output: nu
   'gemini-2.5-flash': { input: 0.15, output: 0.60 },
   'gemini-1.5-pro': { input: 1.25, output: 5 },
   'gemini-1.5-flash': { input: 0.075, output: 0.30 },
+  // GitHub Copilot (included in subscription, tracked at $0)
+  'gpt-4.1': { input: 0, output: 0 },
+  'gpt-4.1-mini': { input: 0, output: 0 },
+  'claude-sonnet-4': { input: 0, output: 0 },
+  'claude-haiku-4': { input: 0, output: 0 },
   // Local / unknown models
   'default': { input: 0, output: 0 },
   // Bedrock (Meta Llama)