chore(types): annotate four containers mypy could not infer

Each element type is taken from how the container is used rather than
guessed: kept_items is returned from trim_to_fit, whose signature is
already list[Any]; traces collects the dicts built at the append site;
expert_results and tool_outputs are keyed by tool_name (str) and hold
ToolReturnPart.content.

tracing_router.py needed `from typing import Any` added — it had no
import for it, so annotating without that would have traded a
var-annotated error for a name-defined one. Function-body variable
annotations are not evaluated at runtime, so this would not have raised;
mypy was the only thing that would have caught it.

90 errors -> 86.

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

src/core/tracing_router.py

@@ -7,6 +7,7 @@ Only available when DEBUG=true.
 
 from datetime import UTC
 from pathlib import Path
+from typing import Any
 
 from fastapi import APIRouter, HTTPException
 from fastapi.responses import HTMLResponse, JSONResponse
@@ -82,7 +83,7 @@ async def list_traces(
         reverse=True,
     )
 
-    traces = []
+    traces: list[dict[str, Any]] = []
     for path in trace_files:
         if len(traces) >= limit:
             break