Add 3-phase execution with IPFS cache and hash-based task claiming

New files:
- claiming.py - Redis Lua scripts for atomic task claiming
- tasks/analyze.py - Analysis Celery task
- tasks/execute.py - Step execution with IPFS-backed cache
- tasks/orchestrate.py - Plan orchestration (run_plan, run_recipe)

New API endpoints (/api/v2/):
- POST /api/v2/plan - Generate execution plan
- POST /api/v2/execute - Execute a plan
- POST /api/v2/run-recipe - Full 3-phase pipeline
- GET /api/v2/run/{run_id} - Get run status

Features:
- Hash-based task claiming prevents duplicate work
- Parallel execution within dependency levels
- IPFS-backed cache for durability
- Integration with artdag planning module

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

celery_app.py

@@ -14,7 +14,7 @@ app = Celery(
     'art_celery',
     broker=REDIS_URL,
     backend=REDIS_URL,
-    include=['tasks']
+    include=['tasks', 'tasks.analyze', 'tasks.execute', 'tasks.orchestrate']
 )
 
 app.conf.update(