fix(agent): address code review findings for tool hint enhancement

- C1: Fix IndexError on empty list arguments via _get_args() helper
- I1: Remove redundant branch in _fmt_known
- I2: Export abbreviate_path from nanobot.utils.__init__
- I3: Fix _abbreviate_url negative-budget format consistency
- S1: Move FORMATS to class-level _TOOL_HINT_FORMATS constant
- S2: Add list_dir to FORMATS registry (ls path)
- G1-G5: Add tests for empty list args, None args, URL edge cases,
  mixed folding groups, and list_dir format

nanobot/utils/__init__.py

@@ -1,5 +1,6 @@
 """Utility functions for nanobot."""
 
 from nanobot.utils.helpers import ensure_dir
+from nanobot.utils.path import abbreviate_path
 
-__all__ = ["ensure_dir"]
+__all__ = ["ensure_dir", "abbreviate_path"]