fix(agent): treat a zero-byte credential file as logout

The backend logs the sandbox out by truncating the github env file to zero
bytes. The resolver returned undefined for that shape, letting consumers fall
back to the frozen process-env token and resurrect the previous actor's
credentials. Treat an empty (or whitespace-only) managed file as an explicit
logout (return ""), with regression tests for the zero-byte case.

Author: VojtechBartos

packages/agent/src/utils/github-token.test.ts

@@ -66,6 +66,12 @@ describe("github-token", () => {
       expect(readGithubTokenFromSandboxEnvFile(path)).toBe("");
     });
 
+    it("returns '' (logout) when the managed file is truncated to zero bytes", () => {
+      // The backend logs the sandbox out by writing an empty file, not emptied vars.
+      expect(readGithubTokenFromSandboxEnvFile(writeEnvFile(""))).toBe("");
+      expect(readGithubTokenFromSandboxEnvFile(writeEnvFile("   \n"))).toBe("");
+    });
+
     it("returns undefined when the file carries no token var at all", () => {
       const path = writeEnvFile("PATH=/usr/bin\0HOME=/root\0");
       expect(readGithubTokenFromSandboxEnvFile(path)).toBeUndefined();
@@ -99,6 +105,13 @@ describe("github-token", () => {
       expect(resolveGithubToken(path)).toBe("");
     });
 
+    it("does not resurrect the process-env token when the file is zero bytes (logout)", () => {
+      // The backend's actual logout truncates the file to zero bytes; resolving
+      // must treat that as logout, not fall back to the frozen process env.
+      vi.stubEnv("GH_TOKEN", "ghs_previous_actor");
+      expect(resolveGithubToken(writeEnvFile(""))).toBe("");
+    });
+
     it("falls back to the process env when the file carries no token var", () => {
       vi.stubEnv("GH_TOKEN", "ghs_fromprocess");
       const path = writeEnvFile("PATH=/usr/bin\0");

packages/agent/src/utils/github-token.ts

@@ -28,6 +28,13 @@ export function readGithubTokenFromSandboxEnvFile(
     }
     return "";
   }
+  // The backend logs the sandbox out by truncating this file to zero bytes, so a
+  // successfully-read but empty (or whitespace-only) managed file is an explicit
+  // logout — return "" so the caller does NOT resurrect the previous actor's token
+  // from the frozen launch-time process env. Only an absent file is "unmanaged".
+  if (raw.trim() === "") {
+    return "";
+  }
   const env: Record<string, string> = {};
   for (const entry of raw.split("\0")) {
     const eq = entry.indexOf("=");