Add --group-by repo|type|date for picker and output

[ashishxcode]

What

Today items are grouped by date. Add a --group-by repo|type|date flag (default date) controlling how items cluster in both the picker and the exported worklog.

• date — current behavior
• repo — cluster by repository (big win on multi-repo days)
• type — cluster by commit / PR / review

Notes

• Affects rendering in internal/render/render.go and the picker ordering in internal/tui/picker.go.
• Consider a TUI toggle (e.g. a key to cycle grouping) in addition to the CLI flag.

Relates to the feature brainstorm in chat (#2).