fix: resolve ruler function/module name collision

[koriyoshi2041]

Summary

Fixes #339.

• Export DEFAULT_RUBRIC from art.rewards so users can access it via from art.rewards import DEFAULT_RUBRIC, bypassing the function/module name collision
• Accept rubric=None in both ruler() and ruler_score_group(), resolving to DEFAULT_RUBRIC internally — this lets users reference the default without importing the constant
• Both functions resolve None at their own entry point for defensive consistency

Background

art.rewards.__init__.py imports the ruler() function from ruler.py, which shadows the ruler module. This makes art.rewards.ruler.DEFAULT_RUBRIC (and any other module-level export) inaccessible:

from art.rewards import ruler
ruler.DEFAULT_RUBRIC  # AttributeError: 'function' object has no attribute 'DEFAULT_RUBRIC'

After this change

# Option 1: Import DEFAULT_RUBRIC directly
from art.rewards import DEFAULT_RUBRIC
custom_rubric = DEFAULT_RUBRIC + "\n- Prefer concise answers"

# Option 2: Pass None to use the default
await ruler_score_group(group, rubric=None)

Fully backward-compatible — existing callers passing a string are unaffected.

Test plan

• Verify from art.rewards import DEFAULT_RUBRIC works
• Verify ruler(message_lists, rubric=None) uses the default rubric
• Verify ruler_score_group(group, rubric=None) uses the default rubric
• Verify existing calls with explicit rubric="..." still work
• uv run prek run --all-files passes