chore: make gpt-prompt customisable

[leemeichin]

At the minute the prompt is hard-coded and changing it requires the use of less-ideal approaches like add-advice.

Sometimes the prompt returns undesirable output, for example:

• commit message over 50 chars
• message sometimes prioritises vague summaries instead of more specific ones (e.g. updates config over adds xyz dependency)
• some changes may be unimportant compared to others, so the prompt output is discarded and written by hand instead
• conventional commit may not be used, or may use a customised version
• the size of the prompt trades-off against the size of the diff that can be analyzed
• no wider context of the overall repo so the best it can offer is 'what' and not 'why'

I figured that allowing customisation would be better than trying to micromanage the hard-coded one and publishing releases for every tweak. In that sense, one could have per-project prompts that describe the project/repo and not just the commit format.