fix: distinguish evidence tree lookup failures

backend/src/workspaces/git-repository.ts

@@ -278,8 +278,10 @@ export class GitWorkspaceRepository {
       );
       return stdout;
     } catch (error) {
-      if (invalidObjectMessage && typeof error === "object" && error !== null
+      const stderr = typeof error === "object" && error !== null && "stderr" in error
-        && "code" in error && typeof error.code === "number") {
+        && typeof error.stderr === "string" ? error.stderr : "";
+      if (invalidObjectMessage
+        && /^fatal: path '[^']+' does not exist in '[0-9a-f]{40}'\s*$/u.test(stderr)) {
         throw new WorkspaceRegistryError("workspace_invalid", invalidObjectMessage);
       }
       throw this.sanitizeGitError(error);

backend/test/workspaces-git-repository.test.ts

@@ -142,6 +142,18 @@ test("redacts Git failures while checking an Evidence tree", async () => {
   expect((error as Error).message).not.toContain(fixture.root);
 });
 
+test("classifies repository corruption as unavailable rather than invalid Evidence", async () => {
+  const fixture = await temporaryRemote();
+  const repository = new GitWorkspaceRepository(config(join(fixture.root, "registry"), fixture.remote));
+  await repository.bootstrap();
+  rmSync(join(repository.repoPath, ".git", "objects"), { recursive: true, force: true });
+
+  await expect(repository.assertTreeAtRevision(
+    fixture.initialCommit,
+    "workspace-content/research/evidence",
+  )).rejects.toMatchObject({ code: "git_unavailable", message: "Workspace Git operation failed" });
+});
+
 test("binds Evidence tree validation to old and new content-only commits", async () => {
   const fixture = await temporaryRemote();
   const repository = new GitWorkspaceRepository(config(join(fixture.root, "registry"), fixture.remote));