[BUG]: No warning when cloud-init has already run and user-data changes are ignored

### What happened? **Description:** When a user modifies the `user-data` file on the boot partition after the SD card has already been booted once, cloud-init silently ignores all changes on subsequent boots. This is by design, but it leads to a confusing user experience because there is no warning or indication that the changes will not take effect. **Steps to reproduce:** 1. Flash Raspberry Pi OS Lite to an SD card using Raspberry Pi Imager 2. Boot the Raspberry Pi for the first time 3. Shut down and remove the SD card 4. Modify `user-data` on the boot partition (e.g. add an SSH public key or change the password hash) 5. Re-insert the SD card and boot again 6. The changes are silently ignored — no error, no warning **Expected behavior:** The Raspberry Pi Imager or the OS should warn the user that cloud-init has already run and that changes to `user-data` will not take effect without reflashing the SD card. **Actual behavior:** Changes are silently ignored. This is especially confusing for users who are trying to fix SSH access issues, as they have no way of knowing why their changes are not working. **Suggested improvement:** Add a warning message (either in Raspberry Pi Imager or via a file on the boot partition) indicating that cloud-init has already completed its first-run setup. This would save users from frustrating debugging sessions. **Environment:** - Raspberry Pi Zero W v1.1 - Raspberry Pi OS Lite 32-bit (Bookworm) - Raspberry Pi Imager latest version - macOS (Intel) ### Version 2.0.9 (Default) ### What host operating system were you using? macOS ### Host OS Version 15.7.5 ### Selected OS Raspberry Pi OS Lite 32-bit (Bookworm) ### Which Raspberry Pi Device are you using? Raspberry Pi Zero, Zero W, Zero WH ### What kind of storage device are you using? microSD Card in an internal reader ### OS Customisation - [ ] Yes, I was using OS Customisation when the bug occurred. ### Relevant log output ```shell ```