Support a .worktreeinclude file

[matthewbauer]

This is a file which we use to "include" untracked files in new worktree. Disabled by default, this makes it easier to share files between main repo and worktrees. I set mine to:

.env
.envrc
**/.claude/settings.local.json

See
https://code.claude.com/docs/en/common-workflows#copy-gitignored-files-to-worktrees for how claude’s works.

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