docs: outline single-workspace core rewrite requirements

[benjaminshafii]

Summary

• add a best-guess PRD for rebuilding the OpenWork core app around a single active workspace
• capture the current non-negotiable requirements around session readability, workspace lifecycle, remote connect, and OpenWork server ownership
• identify the current surfaces that can likely be deferred in a simpler rewrite

Validation

• git diff --check

Notes

• This is a docs-only PR; no runtime or UI behavior changed.
• I did not run Docker or Chrome MCP because the task only adds a research/requirements document.