Default to all-time Codex usage

[aaiyer]

This PR changes Codex Wrapped to aggregate stats across your entire Codex history by default, instead of requiring a per-year run.

• Default run (codex-wrapped) now scans all ~/.codex/sessions//... directories and totals sessions/messages/tokens/projects across all time.
• --year/-y continues to generate a year-specific wrapped (and only then applies the “Wrapped available” date gating).
• The header/tweet text now uses an “All Time” period label when no year is provided.
• Activity heatmap shows the last 12 months in all-time mode; year mode remains unchanged.