perf(read_file): make compact gutter the only format; drop HERMES_READ_GUTTER (#35532)

The compact "<n>|content" gutter from #35368 is now the sole behavior.
Removes the HERMES_READ_GUTTER=padded escape hatch and its env lookup —
no legacy fixed-width path to maintain. Padding was pure token overhead
(~48% more tokens than bare content, ~16% more than compact) with no
measured accuracy gain in the original A/B.

- file_operations.py: drop env lookup + os import; gutter always f"{i}|{line}"
- tests: drop the padded env-override test; compact assertions retained

tests/tools/test_file_operations.py

@@ -356,13 +356,6 @@ class TestShellFileOpsHelpers:
         assert "50|continued" in result
         assert "51|more" in result
 
-    def test_add_line_numbers_padded_env_override(self, file_ops, monkeypatch):
-        # Legacy fixed-width format available via HERMES_READ_GUTTER=padded.
-        monkeypatch.setenv("HERMES_READ_GUTTER", "padded")
-        result = file_ops._add_line_numbers("line one\nline two")
-        assert "     1|line one" in result
-        assert "     2|line two" in result
-
     def test_add_line_numbers_truncates_long_lines(self, file_ops):
         long_line = "x" * (MAX_LINE_LENGTH + 100)
         result = file_ops._add_line_numbers(long_line)

tools/file_operations.py

@@ -714,13 +714,9 @@ class ShellFileOperations(FileOperations):
         line-reference / patch / value-lookup / structure tasks (4/4 both),
         while dropping line numbers entirely regressed line-referencing
         (the model hand-counted and was off-by-one, 3/4) — so we keep the
-        numbers, just not the padding. ``HERMES_READ_GUTTER=padded``
-        restores the legacy fixed-width format for anyone who relied on
-        column alignment.
+        numbers, just not the padding.
         """
-        import os as _os
         from tools.tool_output_limits import get_max_line_length
-        padded = (_os.environ.get("HERMES_READ_GUTTER") or "").lower() == "padded"
         max_line_length = get_max_line_length()
         lines = content.split('\n')
         numbered = []
@@ -728,7 +724,7 @@ class ShellFileOperations(FileOperations):
             # Truncate long lines
             if len(line) > max_line_length:
                 line = line[:max_line_length] + "... [truncated]"
-            numbered.append(f"{i:6d}|{line}" if padded else f"{i}|{line}")
+            numbered.append(f"{i}|{line}")
         return '\n'.join(numbered)
     
     def _expand_path(self, path: str) -> str: