Add a stats/summary command

[kacheo]

Problem

There's no way to see aggregate activity over time — commit/PR counts, how consistently you're journaling, or which areas you've been working in.

Proposal

Add devlog stats [range] reporting over a date range:

• Commits and PRs per day (and totals)
• Active-day count / streaks
• Tag breakdown

Support --from/--until (mirroring show) and --json.

Note

There's no index — this would be a full directory scan per invocation. Fine at personal-journal scale; worth a comment so it's a conscious tradeoff.