Rename tasks.py to legacy_tasks.py to avoid conflict with tasks/ package

The tasks/ directory for 3-phase execution was shadowing the old tasks.py.
Renamed to legacy_tasks.py and updated all imports.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

render.py

@@ -12,7 +12,7 @@ import argparse
 import json
 import sys
 
-from tasks import render_effect
+from legacy_tasks import render_effect
 
 # Known asset hashes
 ASSETS = {