Support npx @ccusage/codex@latest last --day 7

[Cheese-Yu]

Add support for querying usage over the last N days (excluding today), for example:

npx @ccusage/codex@latest last --day 7

This should return usage from the previous 7 full days (yesterday going backwards), excluding the current day.

Expected Behavior:

• --day N returns data from (today - N) to (yesterday)
• Today is always excluded to avoid partial data

Example:
If today is March 25, --day 7 → March 18–24

This would help with consistent short-term usage analysis.