ci: add ICT build hardening and remote flash/validation to Jenkinsfile

Extend the e2e CI Jenkinsfile with:
- ICT prerequisite hardening: Go 1.24.0+ check, universe/apt handling,
  and a chown of ict-tool after the root-owned sudo build so artifact
  discovery does not hit permission errors.
- Copy Artifacts to Flashing Host stage over scp (sshpass), with
  TARGET_NODE_IP/USER/PASSWORD and TARGET_DEST_DIR parameters.
- Flash Target Node stage: extract artifact, inject proxy + SSH key into
  config-file, verify the target device via lsblk, run
  bootable-usb-prepare.sh, select the USB UEFI boot entry from
  efibootmgr, set it as BootNext, and reboot.
- Verify Target Boot and Post-Boot Validation (README Phase 3) stages.
- BUILD_ONLY gate (default on) so a plain build never touches a remote
  host; switch agent label to fed-node2.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ShrutiSS21