Move the worktree picker to the title bar + make it always visible (#54183)

This PR makes Zed only have one worktree picker, as opposed to a flavor
of it in the title bar and another in the agent panel. It then moves it
to the title bar, making it always present, so that its trigger is
separate from the branch picker (which now contains only two views:
branches and stashes). For the worktree picker, I'm mostly favoring the
behavior we've introduced in the agent-panel-flavored version.

It also updates the title bar settings migration to use the JSON
`migrate_settings` helper instead of a shallow Tree-sitter rewrite, so
old `show_branch_icon = true` values are promoted to
`show_branch_status_icon = true` across root, platform, release-channel,
and profile settings scopes.

- [x] Move worktree creation logic to the `git_ui` crate to make this
more generic and less agent-specific
- [x] Double-check the remote use case and ensure nothing broke there
- [x] Improve the UX for the detached HEAD state; better invite people
to create a branch
- [x] Migrate `show_branch_icon = true` to `show_branch_status_icon =
true` across nested settings scopes

Suggested .rules additions

When migrating renamed settings keys that can appear in platform
overrides, release-channel overrides, or profiles, prefer the JSON
`migrations::migrate_settings` helper over shallow Tree-sitter key
rewrites unless tests explicitly cover every nested scope that can
contain the key.

Release Notes:

- Improved migration of the title bar branch status icon setting.

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>
Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

docs/src/visual-customization.md

@@ -118,7 +118,7 @@ To disable this behavior use:
 ```json [settings]
   // Control which items are shown/hidden in the title bar
   "title_bar": {
-    "show_branch_icon": false,      // Show/hide branch icon beside branch switcher
+    "show_branch_status_icon": false, // Show git status on branch icon
     "show_branch_name": true,       // Show/hide branch name
     "show_project_items": true,     // Show/hide project host and name
     "show_onboarding_banner": true, // Show/hide onboarding banners