Fix next steps to use npx instead of global codegraph command

Users running via npx don't have the codegraph command installed
globally. Update installer message and README to use
npx @colbymchenry/codegraph init -i instead.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/installer/banner.ts

@@ -121,7 +121,7 @@ export function showNextSteps(location: 'global' | 'local'): void {
   if (location === 'global') {
     console.log(chalk.dim('  Quick start:'));
     console.log(chalk.dim('    cd your-project'));
-    console.log(chalk.cyan('    codegraph init -i'));
+    console.log(chalk.cyan('    npx @colbymchenry/codegraph init -i'));
   } else {
     console.log(chalk.dim('  CodeGraph is ready to use in this project!'));
   }