- AgentDefinition gains base_url and workspace_path fields (optional)
- load_agent_registry parses them from matrix-agents.yaml
- _build_platform_from_env uses agent.base_url per agent (falls back to AGENT_BASE_URL)
- _agent_workspace_root() resolves workspace per agent from registry
- _materialize_incoming_attachments saves files to agent workspace_path/incoming/
- send_outgoing accepts workspace_root param; reads outgoing files from agent workspace_path
- dispatch loop computes workspace_root from room agent_id and passes to _send_all
- config/matrix-agents.yaml and example updated with base_url and workspace_path
