Update store_media_file to use workspace when available:
- Save files to workspace instead of temp exec_file dir
- Return workspace:// references instead of base64 data URIs
- Handle workspace:// input references (read from workspace)
- Pass session_id to WorkspaceManager for session scoping
- Prevents context bloat (100KB file = ~133KB as base64)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>