This PR updates ghost snapshotting to avoid capturing oversized
untracked artifacts while keeping undo safe. Snapshot creation now
builds a temporary index from `git status --porcelain=2 -z`, writes a
tree and detached commit without touching refs, and records any ignored
large files/dirs in the snapshot report. Undo uses that metadata to
preserve large local artifacts while still cleaning up new transient
files.
0d9801d448