Build the document journal (history chain)

Store a Git-like chain (Merkle tree) of history events and add the ability to navigate in it.

No files, tests, or entry points are named. Begin by locating the document history and navigation code, then review how history events are currently stored and traversed. Done means the project stores a Git-like history chain for document events and supports navigating through it.

索引モデルが issue の本文から書いたものです。