Add `vers whoami` command

[AlephNotation]

Feature request

There's no way to check the current authentication state, user identity, or organization from the CLI. A vers whoami command would help with debugging and orientation.

Suggestion

$ vers whoami
User:    tynan
Org:     hdresearch
API Key: vers_...4f2b (valid)
API URL: https://api.vers.dev

$ vers whoami --format json
{
  "user": "tynan",
  "org": "hdresearch",
  "api_key_prefix": "vers_...4f2b",
  "api_url": "https://api.vers.dev"
}

Impact

Useful for agents to verify auth is configured correctly before running a workflow, and for debugging when operations fail with auth errors.