split: fails when planned file is untracked/unstaged

Summary

coco commit --split fails when a file included in the generated split plan is itself untracked or has unstaged changes. This creates a chicken-and-egg problem when coco's own config file (.coco.json) is part of the commit.

Steps to Reproduce

1. Have a repo with staged changes and an untracked .coco.json file
2. Run coco commit --split
3. Coco generates a split plan that includes .coco.json as one of the files to commit
4. User confirms the plan
5. Coco errors out:

Error: Cannot apply split plan because these files also have unstaged or untracked changes: .coco.json

Expected Behavior

Coco should be able to commit files that are in the split plan even if they're currently untracked. If a file is part of the plan, coco should stage it before applying the commit rather than treating it as a conflict.

Possible Solutions

• Auto-stage files that are part of the approved plan before applying commits
• Skip the dirty-check for files that are explicitly included in the plan
• Exclude untracked files from the plan if they can't be handled, with a clear message

Environment

• coco version: latest
• OS: macOS
• Node: v22.14.0

Start by tracing the coco commit --split command and the dirty-check that produces the reported conflict for .coco.json; reproduce it with staged changes and an untracked or unstaged planned file. Done means a file explicitly included in the approved split plan can be committed without triggering the unstaged or untracked conflict.