fix: eliminate shell noise from terminal output with fence markers

- Wrap commands with unique fence markers (printf FENCE; cmd; printf FENCE)
  to isolate real output from shell init/exit noise (oh-my-zsh, macOS
  session restore/save, docker plugin errors, etc.)
- Expand _clean_shell_noise to cover zsh/macOS patterns and strip from
  both beginning and end (fallback when fences are missing)
- Fix BSD find compatibility: fallback to simple find when -printf
  produces empty output (macOS)
- Fix test_terminal_disk_usage: use sys.modules to get the real module
  instead of the shadowed function from tools/__init__.py
- Add 13 new unit tests for fence extraction and zsh noise patterns

tools/file_operations.py

@@ -848,8 +848,8 @@ class ShellFileOperations(FileOperations):
         
         result = self._exec(cmd, timeout=60)
         
-        if result.exit_code != 0 and not result.stdout.strip():
-            # Try without -printf (BSD find compatibility)
+        if not result.stdout.strip():
+            # Try without -printf (BSD find compatibility -- macOS)
             cmd_simple = f"find {self._escape_shell_arg(path)} -type f -name {self._escape_shell_arg(search_pattern)} " \
                         f"2>/dev/null | head -n {limit + offset} | tail -n +{offset + 1}"
             result = self._exec(cmd_simple, timeout=60)