Remove the ability to add and remove individual folders from project
groups in the sidebar.

## Changes

**Sidebar project group menu:**
- When there are 2+ local project groups, the ellipsis menu shows "Open
Project in New Window" and "Remove Project" (no separators, no folder
listing)
- When there's only one group or it's remote, the ellipsis menu is
replaced with a close X button that removes the project directly

**Project picker labels:**
- "Open Local Project" → "Open Local Folders"
- "Open Remote Project" → "Open Remote Folder"
- "Add Local Project" → "Add Local Folders"
- "Add Remote Project" → "Add Remote Folder"

**Dead code removal:**
- `MultiWorkspace::remove_folder_from_project_group`
- `MultiWorkspace::prompt_to_add_folders_to_project_group`
- `MultiWorkspace::add_folders_to_project_group`
- `MultiWorkspace::update_project_group_key`
- `MultiWorkspaceEvent::ProjectGroupKeyUpdated`
- `Sidebar::move_threads_for_key_change`
- `ThreadMetadataStore::change_worktree_paths_by_main`
- Associated tests for removed functionality

Per-workspace thread regrouping (when a workspace's worktree paths
change) continues to work via the existing `WorktreePathsChanged` →
`move_thread_paths` flow. The existing
`test_worktree_add_only_regroups_threads_for_changed_workspace` test
(renamed and strengthened) verifies that only the changed workspace's
threads are regrouped, while sibling worktree threads remain in their
original group.

Release Notes:

- Improved sidebar project group menu to show a simple close button for
single-project windows and a streamlined two-item menu for multi-project
windows.