Setting to change the "New worktree" behavior for all sessions

It is very annoying that sometimes I just start a new session and it takes minutes to start because it is creating a new worktree. There should be a global setting to change the default to local and then I'll pick when I want a new worktree.

The issue names no files, tests, or entry points. Start by locating session creation and the existing worktree behavior, then find where session defaults are stored or applied. Done means a global setting defaults new sessions to local work and still lets users choose a new worktree when needed.

索引モデルが issue の本文から書いたものです。