feat: enable concurrent E2E test execution via GitHub Actions matrix strategy

- Add TEST_MODEL env var support to index.ts for single-model testing
- Configure code-qa.yml with matrix strategy for 3 AI models
- Each model runs in parallel in its own isolated VSCode instance
- This eliminates race conditions from shared API state
- Local development still tests all models sequentially

apps/vscode-e2e/src/suite/index.ts

@@ -147,8 +147,9 @@ export async function run() {
 	}
 
 	// Print summary
+	const summaryTitle = modelsToTest.length === 1 ? "SINGLE-MODEL TEST SUMMARY" : "MULTI-MODEL TEST SUMMARY"
 	console.log(`\n${"=".repeat(60)}`)
-	console.log(`  MULTI-MODEL TEST SUMMARY`)
+	console.log(`  ${summaryTitle}`)
 	console.log(`${"=".repeat(60)}`)
 
 	for (const result of results) {
@@ -162,6 +163,7 @@ export async function run() {
 	console.log(`${"=".repeat(60)}\n`)
 
 	if (totalFailures > 0) {
-		throw new Error(`${totalFailures} total test failures across all models.`)
+		const modelContext = modelsToTest.length === 1 ? `model ${modelsToTest[0]}` : "all models"
+		throw new Error(`${totalFailures} total test failures for ${modelContext}.`)
 	}
 }