Add changelog command for git history extraction

[ygpark80]

Overview

Add a bump changelog subcommand that collects git history between version tags and outputs structured data.

Deferred from #1 to keep the initial scope focused on bump version.

Proposed CLI

bump changelog [OPTIONS]
  --from-tag <tag>      Start of range (default: latest v* tag)
  --to <ref>            End of range (default: HEAD)
  --output <path>       Write to file (default: stdout)
  --format <fmt>        markdown | json | text (default: markdown)

Output includes

• Commit list (hash, author, date, message)
• Diff stat (files changed, insertions, deletions)
• List of changed files

Use case

Feed structured changelog data to coding agents for generating release notes and commit messages.