chore(gitignore): ignore local AI agent and planning artifacts

Add the following local-only files/dirs to .gitignore so they do not
pollute the working tree status:

  .claude/      # Claude Code per-repo workspace
  AGENTS.md     # local agent guidelines (not project doc)
  CLAUDE.md     # local Claude config (not project doc)
  plan.md       # local planning scratchpad

These are user-side scaffolding files generated by AI coding assistants
and should never be committed to this repository.

Author: jfb8856606

.gitignore

@@ -39,3 +39,9 @@ adapter/syscall/helloworld_stack_epoll
 adapter/syscall/helloworld_stack_epoll_kernel
 adapter/syscall/helloworld_stack_epoll_thread_socket
 adapter/syscall/helloworld_stack_thread_socket
+
+# AI agent / planning artifacts (local only)
+.claude/
+AGENTS.md
+CLAUDE.md
+plan.md