refactor(project): drop persisted Project.sandboxes column

The Project row stored a sandboxes string array that duplicated what git
already tracks. The /experimental/worktree GET handler now reads from
`Worktree.list()` (git worktree list --porcelain) instead, the write-side
addSandbox/removeSandbox bookkeeping is gone, and the column is dropped
from the schema with a Drizzle migration. The app maps the live list onto
project.worktrees client-side instead of reading project.sandboxes.