chore: add CLI CI test

[mattzh72]

Description

Add CLI CI test. This performs a basic test of retrieving an existing agent, and checks the the initial output is valid. A couple of notes:

• We have a pytest fixture that backs up and deletes the user's .letta, and then swaps the backup copy to the .letta directory after the test is done
• We rewrite the credentials file dynamically to prevent API key leakage
• We factor out all the decorative emojis to constants so we can ensure test consistency

Testing

Ran the test locally and confirmed CI passes.

[sarahwooders]

lgtm!