Add letta --logout flag to clear stored auth credentials #1223
Description
When a Letta API key is revoked or rotated, there's currently no headless way to clear the stored auth token. Users have to manually find and delete the credentials file, which isn't documented.
Reported by: grendelousjason - their API key became invalid and they had no way to re-authenticate from the CLI without manually locating the credentials file or setting LETTA_API_KEY as an env var.
Request: Add a letta --logout flag (or letta logout subcommand) that clears the stored API key/auth token so the user is prompted to re-authenticate on next launch.
Why useful:
Key rotation/revocation is a normal workflow
No current headless path to re-auth (interactive /connect only handles LLM providers)
Would mirror standard CLI auth patterns (e.g. gh auth logout, npm logout)