Add gitignore rules to exclude LLM instruction files (#35076)

Similar to how we have ignores for other tooling (eg vscode & IntelliJ)
we shouldn’t include these files in our repo. If they get added then
we’d have to maintain them and keep them up to date, and personally
there are too many tools to do that for.

.gitignore

@@ -109,3 +109,15 @@ prime/
 
 # Manpage
 /man
+
+# Ignore AI/LLM instruction files
+/.claude/
+/.cursorrules
+/.cursor/
+/.goosehints
+/.windsurfrules
+/.github/copilot-instructions.md
+/AGENT.md
+/CLAUDE.md
+/llms.txt
+