Add a code_executor agent tool with a run_code action that runs agent-provided
code in the per-conversation sandbox session and captures produced files as
artifacts. Inputs are materialized only from artifacts the caller can access
(parent-scoped); produced files are stored under the user's namespace with
server-computed size and sha256, and the storage write is ordered last in the
transaction so a failure cannot orphan bytes. Output is a compact payload with
no raw bytes, and the produced artifact lights up the existing tool artifact
rail. Execution honors a wall-clock timeout and an agent-selectable session TTL
clamped by the global cap, and the action can be gated behind approval.
Tool-call argument logging is redacted so code bodies are not written to logs.
