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/path.py

@@ -89,7 +89,8 @@ def _abbreviate_url(url: str, max_len: int = 40) -> str:
 
     budget = max_len - len(domain) - len(basename) - 4  # "…/" + "/"
     if budget < 0:
-        return domain + "/\u2026" + basename[: max_len - len(domain) - 4]
+        trunc = max_len - len(domain) - 5  # "…/" + "/"
+        return domain + "/\u2026/" + (basename[:trunc] if trunc > 0 else "")
 
     # Build abbreviated path
     kept: list[str] = []