VCS branch switcher

[joelmoss]

Wherever the branch/bookmark is mentioned (terminal pane footer, VCS sidebar header), allow a click to open a branch selector, and to click any branch in that list to switch the current branch in the selected workroom.

[joelmoss]

Conflicts with a decision made 5 days before this issue was filed. Commit a9838620 ("add a VCS toolbar to the top of the inspector") deliberately removed in-place branch switching from VCSToolbar.swift, with this doc comment on VCSBranchSegment:

Workrooms are branches — a workroom's identity is its directory name, with no branch field anywhere in the config — so switching a workroom's branch would make its name permanently wrong, and the jj equivalent (jj new <bookmark>) was reproduced removing a file from the working copy and orphaning the workroom's commit. With switching gone there is no action left that belongs on the branch name, so this is a label: no Button, no hover well, no chevron.

Two concrete blockers found for literal in-place switching, as requested here:

1. git — internal/config has no branch field. A workroom's directory name is its identity (set at git worktree add -b time). Switching branch in place leaves the dir name permanently stale, with nothing to reconcile it.
2. jj — jj new <bookmark> onto the bookmark the toolbar displays was measured to remove a file from the working copy and orphan the workroom's commit as a nameless dangling head, silently (no UI signal).

TODOS.md:929-959 already tracks the sanctioned successor: "New workroom from branch…" — pick a local/remote branch from the toolbar's branch segment (or ⌘O) and create a new workroom checked out to it, instead of mutating an existing one. That reuses:

• Views/OpenWorkroomDialog.swift — searchable, project-grouped picker to model the UI on
• Go CLI's workroom create — already accepts a branch arg
• CLIVCSWriter.gitRemoteRefsArgs — cheap remote-ref enumeration (git); no jj equivalent exists yet for listing bookmarks user-facing (only a private jjBookmarkListArgs/parseJJBookmarks pair in VCSWriting.swift used internally for ahead/behind, not exposed)

No VCS-layer API exists today for either listing branches/bookmarks for a picker, or switching/checking out — both would be new work regardless of which direction this goes.

Recommend closing this in favor of (or retitling to) the TODOS.md "New workroom from branch…" item, unless there's a reason to revisit the in-place-switch decision (e.g. the jj orphan bug gets root-caused and fixed, and a config branch field / rename-on-switch policy gets designed for git).