File references are now checked one by one and checked again before sending

Claude Code checks file references, which point to files to be attached, before using them. It now handles several references in one call, and each error says which reference it is about. Only one reference used to be checked. When the references are sent, each one is compared with what was checked earlier. Any difference is reported as changed after the check, with one of these reasons:

• reference_added, a reference that was not there at check time
• reference_changed, a reference that differs from what was checked
• reference_removed, a reference that was dropped
• reference_gone, a reference that no longer exists

The checks for Windows file paths now apply to every reference.