chore: ignore .slack/apps.json files in monorepo gitignore

Prevents accidentally committing App IDs from maintainer environments.
This top-level gitignore is not part of the app templates.

Author: mwbrooks

.gitignore

@@ -6,3 +6,7 @@
 
 # Git
 .worktrees/
+
+# Slack App Templates
+*/.slack/apps.json
+*/.slack/apps.dev.json