fix(backup): handle files with pre-1980 timestamps

ZipFile.write() raises ValueError for files with mtime before 1980-01-01
(the ZIP format uses MS-DOS timestamps which can't represent earlier dates).
This crashes the entire backup. Add ValueError to the existing except clause
so these files are skipped and reported in the warnings summary, matching the
existing behavior for PermissionError and OSError.

hermes_cli/backup.py

@@ -201,7 +201,7 @@ def run_backup(args) -> None:
                 else:
                     zf.write(abs_path, arcname=str(rel_path))
                     total_bytes += abs_path.stat().st_size
-            except (PermissionError, OSError) as exc:
+            except (PermissionError, OSError, ValueError) as exc:
                 errors.append(f"  {rel_path}: {exc}")
                 continue
 

tests/hermes_cli/test_backup.py

@@ -702,6 +702,34 @@ class TestBackupEdgeCases:
         # Zip should still be created with the readable files
         assert out_zip.exists()
 
+    def test_pre1980_timestamp_skipped(self, tmp_path, monkeypatch):
+        """Backup skips files with pre-1980 timestamps (ZIP limitation)."""
+        hermes_home = tmp_path / ".hermes"
+        hermes_home.mkdir()
+        (hermes_home / "config.yaml").write_text("model: test\n")
+
+        # Create a file with epoch timestamp (1970-01-01)
+        old_file = hermes_home / "ancient.txt"
+        old_file.write_text("old data")
+        os.utime(old_file, (0, 0))
+
+        monkeypatch.setenv("HERMES_HOME", str(hermes_home))
+        monkeypatch.setattr(Path, "home", lambda: tmp_path)
+
+        out_zip = tmp_path / "out.zip"
+        args = Namespace(output=str(out_zip))
+
+        from hermes_cli.backup import run_backup
+        run_backup(args)
+
+        # Zip should still be created with the valid files
+        assert out_zip.exists()
+        with zipfile.ZipFile(out_zip, "r") as zf:
+            names = zf.namelist()
+            assert "config.yaml" in names
+            # The pre-1980 file should be skipped, not crash the backup
+            assert "ancient.txt" not in names
+
     def test_skips_output_zip_inside_hermes(self, tmp_path, monkeypatch):
         """Backup skips its own output zip if it's inside hermes root."""
         hermes_home = tmp_path / ".hermes"