Add an activity calendar heatmap to the stats command

[jongio]

Summary

Adds a --calendar flag to dispatch stats that renders a GitHub-style contribution heatmap of sessions per day, so you can see when work happened and spot streaks and gaps.

Behavior

• dispatch stats --calendar prints a seven-row weekday grid of shaded intensity blocks with a Less ... More legend.
• Days are grouped by creation date (UTC); every day between the first and last active day is shown so gaps render as empty cells.
• Intensity buckets (0-4) scale to the busiest day in the range.
• The default text and JSON output are unchanged; the calendar is added only when --calendar is passed.
• Honors the existing --repo, --branch, --since, and --until filters.

Changes

• cmd/dispatch/stats.go: --calendar flag, buildActivityCalendar aggregation, intensityBucket, and grid rendering. The report gains an optional calendar field (omitted by default).
• cmd/dispatch/main.go: documents the flag in usage.
• Unit tests cover the date-to-per-day aggregation, intensity buckets, gap filling, and both text and JSON output paths.
• README documents the stats command and the flag.

Closes #222