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/frontends/tui/commands.ts

@@ -8,6 +8,7 @@ export type Command =
   | { type: 'usage' }
   | { type: 'model'; name?: string }
   | { type: 'backend'; provider?: string }
+  | { type: 'login'; provider?: string }
   | { type: 'transfer'; target: string }
   | { type: 'message'; content: string };
 
@@ -74,6 +75,15 @@ export function parseCommand(input: string): Command | null {
     return { type: 'transfer', target };
   }
 
+  // Login
+  if (trimmed === '/login') {
+    return { type: 'login' };
+  }
+  if (trimmed.startsWith('/login ')) {
+    const provider = trimmed.slice('/login '.length).trim();
+    return { type: 'login', provider: provider || undefined };
+  }
+
   // Regular message
   return { type: 'message', content: trimmed };
 }
@@ -84,6 +94,7 @@ Commands:
   /help, /?              Show this help
   /model [name]          Show or switch model (local, default, fast, complex)
   /backend [provider]    Show or switch local backend (ollama, llamacpp)
+  /login [provider]      Authenticate with GitHub
   /reset, /clear, /new   Clear conversation history
   /compact               Compact conversation history
   /usage                 Show token usage and estimated cost
@@ -109,6 +120,7 @@ export const SLASH_COMMANDS = [
   '/status',
   '/fullscreen',
   '/fs',
+  '/login',
   '/transfer',
   '/quit',
   '/exit',
@@ -127,6 +139,7 @@ export const COMMAND_TOOLTIPS: Record<string, string> = {
   '/status': 'Show session info and token usage',
   '/fullscreen': 'Switch to fullscreen mode',
   '/fs': 'Switch to fullscreen mode',
+  '/login': 'Authenticate with GitHub (OAuth device flow)',
   '/transfer': 'Transfer session to another frontend',
   '/quit': 'Exit TUI',
   '/exit': 'Exit TUI',