feat: resolve real user ID in analytics via UserStore

[theFong]

Summary

• Adds a UserStore interface to the analytics package with GetCurrentUserID() (string, error)
• SetUserStore() is called once during init with noLoginCmdStore, which already implements this method
• GetOrCreateAnalyticsID() now tries the store first for the real user ID, falling back to the anonymous UUID
• Simplifies PersistentPostRunE — no more manual user resolution in cmd.go
• Logout is handled naturally: once credentials are cleared, the store call fails and we fall back to the anon ID

Test plan

• Verify brev ls still reports analytics with user ID when logged in
• Verify analytics falls back to anonymous UUID when logged out
• Verify brev logout followed by a command uses the anonymous ID