[idea] Expose branch and ahead/behind in the workspace API

[staude]

idea / problem

herdr already computes the git branch and ahead/behind for each workspace (WorkspaceGitStatus, cached_git_ahead_behind) and shows them in the sidebar's default second row (e.g. main ↑1). API clients cannot get them: workspace.list, workspace.get, session.snapshot and the workspace_updated event carry no git fields. worktree.list gives the branch per worktree, but no ahead/behind, and a client has to map spaces to worktrees itself.

requested change

Add optional branch and ahead_behind: {ahead, behind} to WorkspaceInfo, filled from the existing cache, and emit workspace_updated when they change. Leave them out for spaces outside git.

why you want this

I am building a remote client that mirrors the herdr sidebar on iPad and Mac over SSH streamlocal. Its key is restricted to the socket (no shell), so it cannot run git on the host. With these fields the client could show the same branch ↑N ↓M as herdr's own sidebar, without polling worktree.list and guessing which worktree a space belongs to.

Originally filed as #5362, which was closed with a pointer to this category.