Framework workspaces gain multiple isolated Git heads

The everruns crate now has public workspace and environment types, so several sessions can work on one repository without sharing files. A workspace has named heads; each session binds permanently to exactly one head and resumes on that same head.

With the local feature, a local Git-worktree provider creates each head as a worktree from a revision. Heads are isolated by default; opt into a shared head explicitly. Fork, checkpoint, archive and destroy are explicit calls, and dropping a value never deletes a worktree. The provider interface is open for other backends.

See Workspaces and environments and #3166.