feat(core): add WorkspaceContext

[jlongster]

Breaking my latest work into small PRs. This one adds a new context that code that use to get the workspaceID

I didn't want to introduce a new concept initially, and tried to add a workspaceID getter to Instance instead. That turned out to cause all sorts of confusion though: sometimes only directory is available and no workspace id (that won't be the case long-term, but we will need a phase of migration), and the other way around too. In particular, it caused a lot of confusion due to caching of the context value internally; do we cache based on directory or workspace id? It should be workspace id, but sometimes that isn't available

Pulling it out into a new WorkspaceContext simplified everything. This is purely internal and publicly people won't be dealing with both instances and workspaces.

Similar to directory, this adds a new way to "route" requests: you can specify a workspace url param or use the x-opencode-workspace header to scope your action to a specific workspace. Eventually, everything will be a workspace, even your default local repo.

I have more work which builds on this to rework the router in control-plane to leverage it and will open more once this lands.