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)