Add --stats flag to show session statistics

[Ascendral]

Problem

There's no way to see aggregate statistics about CodeBot usage — total sessions, total tool calls, total tokens used, etc.

Expected behavior

$ codebot --stats
CodeBot AI Usage Stats

  Sessions:     42
  Tool calls:   1,847
  Tokens used:  2.4M
  Total cost:   $12.50
  Last session: 2026-03-17

Files likely involved

• src/cli/args.ts — add --stats flag
• src/cli.ts — add handler
• src/metrics.ts — has MetricsCollector with aggregate data

Acceptance criteria

• codebot --stats prints aggregate usage stats and exits with code 0
• Reads from the existing metrics/audit system
• Should complete in under 1 second