feat: add poly whoami for credential validation and account info

[HelenGreul]

Problem or motivation

There is currently no way to verify that a POLY_ADK_KEY is valid without running a command that requires a project (e.g. poly pull). Developers who have just set up the tool — or who are debugging an auth failure — have no quick way to check: am I authenticated? What account/workspace am I connected to?

This leads to confusing error messages when the key is wrong, expired, or scoped to a different workspace, because the failure surfaces inside a project command rather than at the credential layer.

Proposed solution

Add a poly whoami command that validates the API key and prints account info:

$ poly whoami

  Authenticated as: Jane Smith (jane@example.com)
  Workspace:        Acme Corp
  Region:           us-east
  Key:              poly_k_••••••••••••••••3f9a

If the key is missing or invalid, exit with a clear error:

$ poly whoami
Error: POLY_ADK_KEY is not set. Set it with:
  export POLY_ADK_KEY=<your key>

$ poly whoami
Error: API key is invalid or expired. Contact your PolyAI representative to get a new key.

Alternatives considered

Adding a --check flag to poly init. But whoami is more discoverable, is a standard pattern in developer CLIs (gh auth status, heroku auth:whoami, vercel whoami), and works independently of having a project initialized.

Scope

CLI commands

[bplevin36]

I'll look into this one in the upcoming changes to our API key management