Implement `cost:breakdown` — N-day window with previous-window trend comparison

Implementation issue split out from #107.

Replace the mock-data body in packages/ts-cloud/bin/commands/cost.ts (cost:breakdown command) with real implementation:

• Honor the existing --days <n> option (default 30).
• Query Cost Explorer for last N days and the N days before that to compute the trend column.
• Group by service.

Acceptance

• cloud cost:breakdown and cloud cost:breakdown --days 7 return real numbers.
• --profile flag honored.
• Cached via #106.
• Trend column reflects real comparison, not hardcoded ^ +12%.
• Replaces the cli.warn('not implemented') guard introduced by #107.

Related

• #107 — tactical guard