chore: gitignore Claude Code agent isolation worktrees

The harness creates .claude/worktrees/<id>/ when spawning agents with
isolation: "worktree". These are temporary per-agent clones; they
should never be committed to the parent tree.

Author: claude

.gitignore

@@ -7,3 +7,6 @@ target/
 
 # Apple details
 **/.DS_Store
+
+# Claude Code: agent isolation worktrees (temporary, per-agent)
+.claude/worktrees/