Add engine/history.go with GetTaskHistory and diff-style FormatTaskHistory
that compares consecutive change_log entries to show only what changed.
Add cmd/log.go command for full task history. Integrate last 5 history
entries into FormatTaskDetail (info view) as a "Recent Changes" section.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
