feat(cli): make promote status inspect campaign snapshots

[aiirvizionz]

Part of #133.

The documented sh1pt promote status command currently returns a stub. This issue narrows the first implementation slice to a credential-free, read-only status view over the local .sh1pt/campaigns.json snapshot:

• aggregate spend, impressions, clicks, installs, and conversions;
• support --platform filtering and --json output;
• show an actionable empty-state path when no snapshot exists;
• avoid launching campaigns or requiring social-account credentials.

[ralyodio]

conclits