[change] Improve system prompt of changelog bot

[pushpitkamboj]

The changelog bot prompt should be improved to generate more accurate and better formatted changelog suggestions. In particular, it should be more explicit about the expected commit-message-like structure, plain #issue references, and the introductory text shown in the PR comment.

[openwisp-companion]

This issue has been automatically assigned to @pushpitkamboj who opened PR #657 to address it. 🎯