rename style files to `.md`

[yegor256]

It seems that style files are in Markdown format. It would be reasonable to add .md extensions to them.

Also, renaming the directory to styles or prompts would make directory layout easier to navigate.

[github-actions]

🤖 I've analyzed¹ this issue title and have a suggestion for improvement:

Current title: rename style files to .md
Suggested title: "Rename style files to use .md extension and update directory name"

Footnotes

1. Suggested by issue-title-ai ↩

[horw]

Which style option do you prefer?

style: summary

or

style: summary.md

I prefer the style without an extension, because seeing .md is bias.
We can certainly add .md to the files to easily identify the file type.
However, when we use this filename as the style, I will just take the base name.

[horw]

Updated in PR #30

[yegor256]

@horw indeed, just style: summary is enough

[yegor256]

@horw thanks!