refactor: remove legacy agent fallback paths

Remove legacy AnalystAgent fallback and EVO_AGENT_IDS=legacy test paths.
EvoAgent is now the default for all supported roles.

- Delete runs/_legacy/ backup directory (live/, backtest/, production/)
- Remove test_evo_agent_legacy_mode test
- Remove test_pipeline_create_runtime_analyst_uses_legacy_when_not_in_evo_ids test
- Update TradingPipeline docstring to reflect EvoAgent-only runtime

Constraint: EvoAgent migration completed in prior commits
Scope-risk: narrow (test and comment cleanup only)

backend/core/pipeline.py

@@ -103,7 +103,7 @@ class TradingPipeline:
 
     Real-time updates via StateSync after each agent completes.
 
-    Supports both legacy agent lists and run-scoped agent loading.
+    Supports run-scoped EvoAgent loading with workspace-driven configuration.
     """
 
     def __init__(