ai-usage Adds Signed Team Relay

This is a pragmatic observability layer for teams juggling multiple AI subscriptions, with privacy controls that are useful but not absolute.

• –Reads existing local tool data without consuming additional model quota or requiring provider API keys
• –Signs fixed-shape snapshots while sealing names, emails, and errors with the team key
• –The relay still sees provider, plan, quota metadata, machine counts, and connecting IP addresses
• –Teams can override the default relay with `ai-usage relay set URL` or self-host one through Vercel, Docker, or the CLI
• –Offline mode keeps the tool useful for individuals who do not need shared reporting