test(perf): use git ls-files fast path for guardrail source scan

src/test-utils/runtime-source-guardrail-scan.ts

@@ -1,3 +1,4 @@
+import { execFileSync } from "node:child_process";
 import fs from "node:fs/promises";
 import path from "node:path";
 import { listRuntimeSourceFiles } from "./repo-scan.js";
@@ -63,21 +64,42 @@ async function readRuntimeSourceFiles(
   return output.filter((entry): entry is RuntimeSourceGuardrailFile => entry !== undefined);
 }
 
+function tryListTrackedRuntimeSourceFiles(repoRoot: string): string[] | null {
+  try {
+    const stdout = execFileSync("git", ["-C", repoRoot, "ls-files", "--", "src", "extensions"], {
+      encoding: "utf8",
+      stdio: ["ignore", "pipe", "ignore"],
+    });
+    return stdout
+      .split(/\r?\n/u)
+      .filter(Boolean)
+      .filter((relativePath) => relativePath.endsWith(".ts") || relativePath.endsWith(".tsx"))
+      .filter((relativePath) => !shouldSkipGuardrailRuntimeSource(relativePath))
+      .map((relativePath) => path.join(repoRoot, relativePath));
+  } catch {
+    return null;
+  }
+}
+
 export async function loadRuntimeSourceFilesForGuardrails(
   repoRoot: string,
 ): Promise<RuntimeSourceGuardrailFile[]> {
   let pending = runtimeSourceGuardrailCache.get(repoRoot);
   if (!pending) {
     pending = (async () => {
-      const files = await listRuntimeSourceFiles(repoRoot, {
-        roots: ["src", "extensions"],
-        extensions: [".ts", ".tsx"],
-      });
-      const filtered = files.filter((absolutePath) => {
-        const relativePath = path.relative(repoRoot, absolutePath);
-        return !shouldSkipGuardrailRuntimeSource(relativePath);
-      });
-      return await readRuntimeSourceFiles(repoRoot, filtered);
+      const trackedFiles = tryListTrackedRuntimeSourceFiles(repoRoot);
+      const sourceFiles =
+        trackedFiles ??
+        (
+          await listRuntimeSourceFiles(repoRoot, {
+            roots: ["src", "extensions"],
+            extensions: [".ts", ".tsx"],
+          })
+        ).filter((absolutePath) => {
+          const relativePath = path.relative(repoRoot, absolutePath);
+          return !shouldSkipGuardrailRuntimeSource(relativePath);
+        });
+      return await readRuntimeSourceFiles(repoRoot, sourceFiles);
     })();
     runtimeSourceGuardrailCache.set(repoRoot, pending);
   }