[FEATURE] Headless / exportable /stats (--json and image export)

Problem / use case

/stats only renders inside the interactive TUI — there is no non-interactive output, so it can't be automated (e.g., a weekly scheduled job that posts your stats card to Slack). I had to reverse-engineer ~/.claude/stats-cache.json to reproduce the card.

Proposed

A non-interactive claude stats that supports:

• --json for machine-readable metrics, and
• an image export (e.g. --output png) of the card,

so the exact /stats snapshot can be generated and sent programmatically on a schedule.