docs: Clarify method for change storage when using Coordination Workspaces

[devbytyler]

The documentation for the beta "Coordination Workspaces" feature explains how workspaces link multiple repos for cross-repo planning and coordination. However, it does not clarify a critical point: where are spec changes stored when a change spans multiple repos?

Specifically:

• Is there support for a change folder that spans multiple repos?
• If so, is the change folder stored in one owning repo's openspec/changes/ directory, duplicated across repos, or somewhere in the workspace directory/context store?
• How does OpenSpec expect users to organize multi-repo delta specs and change artifacts?

The current documentation only mentions repo-local openspec/ directories, and the workspace manifest example does not show a location for changes themselves.

Could the docs clarify:

• The intended approach for storing, tracking, and merging changes that touch multiple repos.
• Any best practices or limitations around multi-repo change management.

Thanks for advancing clarity on this beta feature!

Reference: docs/concepts.md