cmd/sgai: extend external MCP workspace workflows

[ucirello]

Summary

• extend the external MCP workspace surface with interactive and continuous starts, pending-question answers, workflow links, workspace logs, exported messages/todos, and fork creation that keeps parent GOAL frontmatter while requiring an explicit title
• normalize external workspace labels, handles, and active-agent metadata so duplicate names, root and fork relationships, and routed workspace links stay consistent across the API and the webapp
• expand Go and Bun coverage for the new external workflow paths, pending-response flows, and workspace-detail state derived from the richer external metadata

Test Plan

• make absorb-sgai