Codex Desktop should guide Git initialization before worktree creation

Codex Desktop worktree creation has poor behavior for newly opened folders that are not initialized Git repositories or have no commits.

Expected behavior:
When a user opens a normal project folder and clicks the UI button to create a worktree/branch, Codex should either:

1. detect that the folder is not a Git repo and offer to initialize it, create or suggest a sensible .gitignore, and make the first baseline commit; or
2. show a clear guided message explaining that the project needs Git initialization and a first commit before worktrees can be created.

Actual behavior:
The UI can hide the worktree button or fail with low-level Git errors such as:

fatal: invalid reference: HEAD

Local reproduction/context:

• A new project folder at /Users/sergey/ai/bene_rus_v2 was not a Git repository at all, so the user could not create a worktree through the expected UI flow.
• An accidental empty parent repository at /Users/sergey/.git also caused child folders to be misdetected as part of that empty repo, so Codex tried to create a worktree from a non-existent HEAD.
• The user expected to open a project folder and use the UI button, not manually diagnose Git state.

Why this matters:
A normal user should not need to understand Git internals, run git init manually, create a .gitignore, make a first commit, or diagnose HEAD/reference errors just to use Codex's worktree feature.

Suggested fix:
Add a preflight check before worktree creation:

• detect whether the selected folder is inside a Git repository;
• detect whether HEAD resolves to a valid commit;
• detect whether the resolved Git root is unexpectedly a parent directory such as the user's home folder;
• if no repo exists, offer an "Initialize Git project" flow;
• if a repo exists but has no commits, offer a "Create initial commit" flow;
• if the repo root is a suspicious parent directory, warn the user and ask whether to use the selected folder as a separate project;
• only run git worktree add after HEAD is valid.

The UI should surface this as a guided setup flow, not as raw Git errors or a missing worktree button.