[FEATURE] Improve credentials experience in the framework

[n1colasf]

Problem

When users run chat, importagent, or migrate commands without credentials configured, the error messages are unhelpful:

• They reference COGSOL_API_BASE instead of the actual credential variables (COGSOL_API_KEY / COGSOL_AUTH_CLIENT_ID)
• They give no indication of what went wrong or how to fix it
• They do not point users to any onboarding resource

This creates a poor first-run experience, especially for new users who have not yet configured their .env file.

Expected Behavior

When credentials are missing, the CLI should:

1. Clearly state which variable is missing
2. Explain where to obtain the credentials
3. Show the exact steps to configure them

Current Behavior

chat command:

COGSOL_API_BASE is required in .env to chat with CogSol.

importagent command:

COGSOL_API_BASE is required in .env to import.

migrate command:

COGSOL_API_BASE is required in .env to run migrations against CogSol APIs.

Core API client (COGSOL_AUTH_SECRET missing):

Missing authentication configuration: COGSOL_AUTH_SECRET

Proposed Solution

Update error messages across chat, importagent, migrate, and cogsol/core/api.py to check for the correct credential variables (COGSOL_API_KEY / COGSOL_AUTH_CLIENT_ID) and display actionable guidance pointing to https://onboarding.cogsol.ai.