feat(cli): add gmail-auth command for OAuth2 token setup

Implements `flynn gmail-auth` to complete the OAuth2 flow that
GmailWatcher references but was never built. Supports local callback
server (default) and --manual paste mode. Adds Gmail health check
to `flynn doctor`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/index.ts

@@ -8,6 +8,7 @@ import { registerConfigCommand } from './config-cmd.js';
 import { registerTuiCommand } from './tui.js';
 import { registerCompletionCommand } from './completion.js';
 import { registerSetupCommand } from './setup.js';
+import { registerGmailAuthCommand } from './gmail-auth.js';
 
 export function createProgram(): Command {
   const program = new Command();
@@ -25,6 +26,7 @@ export function createProgram(): Command {
   registerConfigCommand(program);
   registerCompletionCommand(program);
   registerSetupCommand(program);
+  registerGmailAuthCommand(program);
 
   return program;
 }