- tools/base.py: add current_user_id ContextVar (set by Agent before
  every tool call, cleared after)
- core/agent.py: set current_user_id in run_stream from session.user_id
  and in run_ephemeral from parent_session.user_id; restore in finally
- tools/filesystem.py: _check_path resolves all paths inside
  user_data/<user_id>/ when current_user_id is present; legacy mode
  (no user_id) falls back to FS_ALLOWED_PATHS
- tools/share_file.py: validate source path is inside user sandbox

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>