Generate commit messages from diff

Add instructions to only respond with the commit message and to warn
about any leaked keys. This improves the specificity and security of
the assistant's output.

gitbutler-ui/src/lib/backend/summarizer.ts

@@ -9,6 +9,8 @@ Focus the most important changes.
 Use the present tense.
 Always use semantic commit prefixes.
 Hard wrap lines at 72 characters.
+Only respond with the commit message.
+Warn about leaked keys.
 %{brief_style}
 %{emoji_style}