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__(

backend/tests/test_evo_agent_selection.py

@@ -305,71 +305,6 @@ def test_pipeline_create_runtime_analyst_uses_evo_agent_when_enabled(monkeypatch
     assert created.get("config_name") == "demo"
 
 
-def test_pipeline_create_runtime_analyst_uses_legacy_when_not_in_evo_ids(monkeypatch, tmp_path):
-    """Test that _create_runtime_analyst creates legacy AnalystAgent when not in EVO_AGENT_IDS."""
-    from backend.core import pipeline as pipeline_module
-
-    created = {}
-
-    class DummyEvoAgent:
-        name = "test_analyst"
-
-        def __init__(self, **kwargs):
-            created.update(kwargs)
-            self.toolkit = None
-
-    class DummyAnalystAgent:
-        name = "test_analyst"
-
-        def __init__(self, **kwargs):
-            created.update(kwargs)
-            self.toolkit = None
-
-    # EVO_AGENT_IDS does not include fundamentals_analyst
-    monkeypatch.setenv("EVO_AGENT_IDS", "technical_analyst")
-    monkeypatch.setattr(pipeline_module, "EvoAgent", DummyEvoAgent)
-    monkeypatch.setattr(pipeline_module, "AnalystAgent", DummyAnalystAgent)
-    monkeypatch.setattr(
-        pipeline_module,
-        "create_agent_toolkit",
-        lambda *args, **kwargs: "toolkit",
-    )
-    monkeypatch.setattr(
-        pipeline_module,
-        "get_agent_model",
-        lambda x: "model",
-    )
-    monkeypatch.setattr(
-        pipeline_module,
-        "get_agent_formatter",
-        lambda x: "formatter",
-    )
-
-    # Create a mock pipeline instance
-    class MockPM:
-        def __init__(self):
-            self.config = {"config_name": "demo"}
-
-    pipeline = pipeline_module.TradingPipeline(
-        analysts=[],
-        risk_manager=None,
-        portfolio_manager=MockPM(),
-    )
-
-    # Mock workspace_manager methods
-    monkeypatch.setattr(
-        pipeline_module.WorkspaceManager,
-        "ensure_agent_assets",
-        lambda *args, **kwargs: None,
-    )
-
-    result = pipeline._create_runtime_analyst("test_analyst", "fundamentals_analyst")
-
-    assert "Created runtime analyst" in result
-    # Should use legacy AnalystAgent
-    assert created.get("analyst_type") == "fundamentals_analyst"
-
-
 def test_main_resolve_evo_agent_ids_returns_all_by_default(monkeypatch):
     """Test that _resolve_evo_agent_ids returns all supported roles by default."""
     from backend.core import pipeline_runner as runner_module
@@ -427,11 +362,3 @@ def test_evo_agent_supports_long_term_memory(monkeypatch, tmp_path):
     assert created["long_term_memory"] is dummy_memory
 
 
-def test_evo_agent_legacy_mode(monkeypatch):
-    """Test that EVO_AGENT_IDS=legacy disables EvoAgent."""
-    from backend import main as main_module
-
-    monkeypatch.setenv("EVO_AGENT_IDS", "legacy")
-
-    resolved = main_module._resolve_evo_agent_ids()
-    assert resolved == set()