fix: detect ubuntu-slim runners early and bail out

ubuntu-slim runners (Hosted Compute Agent Docker containers) are
GitHub-hosted but lack the standard USER environment variable set
on full VM-based runners. This causes chownForFolder to fail with
'chown: invalid user: undefined'.

Instead of patching chownForFolder, detect ubuntu-slim early
informative message, matching the existing patterns for isDocker(),
isARCRunner(), and other unsupported runner types.

Fixes #627

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: devantler

src/cleanup.ts

@@ -26,6 +26,11 @@ import { isPlatformSupported, isAgentInstalled } from "./utils";
     return;
   }
 
+  if (isGithubHosted() && process.platform === "linux" && !process.env.USER) {
+    console.log(common.UBUNTU_SLIM_MESSAGE);
+    return;
+  }
+
   if (isARCRunner()) {
     console.log(`[!] ${common.ARC_RUNNER_MESSAGE}`);
     return;

src/common.ts

@@ -148,3 +148,6 @@ export const ARM64_RUNNER_MESSAGE =
 
 export const ARM64_WINDOWS_RUNNER_MESSAGE =
   "Windows ARM runners are not yet supported by Harden-Runner.";
+
+export const UBUNTU_SLIM_MESSAGE =
+  "This job is running on an ubuntu-slim runner (GitHub-hosted Linux container without a standard USER environment variable). Harden Runner is not supported on ubuntu-slim runners. This job will not be monitored.";

src/setup.ts

@@ -64,6 +64,11 @@ interface MonitorResponse {
       return;
     }
 
+    if (isGithubHosted() && process.platform === "linux" && !process.env.USER) {
+      console.log(common.UBUNTU_SLIM_MESSAGE);
+      return;
+    }
+
     var correlation_id = uuidv4();
     var api_url = STEPSECURITY_API_URL;
     var web_url = STEPSECURITY_WEB_URL;