Create worktrees in detached HEAD state, then attempt to check out the
requested branch. If the checkout fails (e.g. the branch is already in
use by another worktree), log a warning and stay in detached HEAD state
instead of showing an error to the user.

This simplifies the worktree creation flow by:
- Removing the occupied-branch fallback logic that would generate random
branch names
- Always using `CreateWorktreeTarget::Detached` for worktree creation
- Attempting branch checkout as a best-effort post-creation step
- Updating the branch picker UI to reflect the new behavior

Release Notes:

- Threads now start git worktrees in detached HEAD state if branch is in
use or unspecified, instead of generating a random branch name.

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Co-authored-by: Eric Holk <eric@zed.dev>