chore: ignore Copilot dogfooding scaffolding in .gitignore

Ignore the directories and files generated by
`specify init --integration copilot` so the dogfooding scaffolding used
during Spec Kit feature development isn't accidentally committed.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mnriem

.gitignore

@@ -50,3 +50,14 @@ docs/dev
 .specify/extensions/.cache/
 .specify/extensions/.backup/
 .specify/extensions/*/local-config.yml
+
+# 
+# The following directories/file are intentionally ignored so that they are not accidentally 
+# committed to the repository. They contain the scaffolding `specify init --integration copilot`
+# does and they are meant for dogfooding Spec Kit during its own feature development.
+#
+.github/agents/
+.github/prompts/
+.github/copilot-instructions.md
+.specify/
+specs/