feature: support for monorepo - `/openspec` not root-level but per-project

[volkb79-2]

In the documentation i see support for a multi-repo setup.

But it seems the monorepo - several projects inside a single repo - is missing support for clean separation of data per-project.

Currently /openspec is created in the repo-root. With multiple projects inside a repo i would expect

/project1/openspec
/project2/openspec