Simplify Spec resolution and add track/untrack commands

[dreikanter]

Summary

• Rename Spec.LiveRoot/SavedRoot to LivePath/SavedPath — the "Root" suffix was misleading for single-file specs
• Make directory classification manifest-driven: a trailing / is now the sole signal for directory-ness; disk inference is removed. Mismatches (dir without slash, file with slash) are now explicit errors in save, install, and status
• Simplify resolveTool: drop the intermediate item struct and dead original field, rename containers to anchors, reduce to a clean two-pass pure function
• Add dotfiles track --tool <name> <path> to add a path to the manifest (must exist on disk; directories detected automatically)
• Add dotfiles untrack --tool <name> <path> to remove a path from the manifest, with --purge to also delete the saved copy
• Add SaveManifest, Expand, and ManifestPath to the dotfiles package
• Bump to 0.2.0 (breaking: trailing slash now required for directory entries)