Allow users to override including space in subspace

### Your use case

#### What would you like to do?

I'd like to have the option to promote a subspace to a parent, or to somehow globally override where it is placed in my local space hierarchy.

#### Why would you like to do it?

[This activitypub space](https://matrix.to/#/#activitypub-community:codelutin.com) contains [this Gitea space](https://matrix.to/#/#gitea-space:matrix.org). I'd joined the latter separately, then it got added to the activitypub space and I couldn't find it for a while.

I guess I understand why the admin did this--it certainly makes sense to connect spaces with common goals--but it feels a bit heavy-handed that all the other communities/projects I follow have their spaces at the top level except for this one, which is now nested. On one hand, I kind of want to encourage adding more spaces to the activitypub space because that makes their projects easy to find. On the other, if that makes all these spaces appear below activitypub, it feels a bit heavy-handed that all of a sudden this other space is now my gateway.

#### How would you like to achieve it?

Genuinely not sure. Maybe some sort of local state override, triggered by dragging-and-dropping a subspace out of its place in the hierarchy? Then someone can still make spaces disappear into others, but I can override that and manually indicate that for my own purposes I want to access that state at the top-level. For simplicity I'd probably not add the ability to drag-and-drop one space into another, unless that's an eventual add flow you plan on supporting.

### Have you considered any alternatives?

I'll probably reach out to the space admin and ask that they remove the space as a whole, maybe only including rooms that directly impact both Gitea and federation. But maybe that just kicks the can down the road, and now I have individual rooms plucked out of the space and inserted elsewhere in the hierarchy?

### Additional context

_No response_

No files, tests, or entry points are identified. Start by locating the existing space hierarchy and inclusion behavior, then determine how a user-local override could coexist with the admin-defined hierarchy. Done would require an agreed interaction and behavior for promoting or repositioning a subspace without changing the server-side structure.

Written by the indexing model from the issue text.