Implement `cost` command — current MTD spend + naive linear projection

Implementation issue split out from #107.

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

• Query Cost Explorer for current month-to-date spend (use CostExplorerClient.getCostByService with start = first of current month, end = today + 1 day).
• Compute naive linear projection: total / days_elapsed * days_in_month.
• Optionally compare to last month for the Change column.

Acceptance

• cloud cost returns real numbers from the connected AWS account.
• --profile flag honored (same plumbing as cost:analyze).
• Caches via the same Cost Explorer cache (#106).
• Replaces the cli.warn('not implemented') guard introduced by #107.

Related

• #107 — tactical guard
• #104 — original cost CLI scope