fix: update sticky reference for commit messages (#1207)

Changes the commit prompt's sticky reference from '#git:staged' to
'#gitdiff:staged' to properly match the expected format for retrieving
staged changes when generating commit messages.

Author: danilohorta

lua/CopilotChat/config/prompts.lua

@@ -204,6 +204,6 @@ return {
 
   Commit = {
     prompt = 'Write commit message for the change with commitizen convention. Keep the title under 50 characters and wrap message at 72 characters. Format as a gitcommit code block.',
-    sticky = '#git:staged',
+    sticky = '#gitdiff:staged',
   },
 }