docs(agent): close #74 matrix + #75 V2 install (themed first boot)

Unattended env matrix flags shipped earlier; default arm V2: live ISO
rebuild + offline install + first-boot themed Boreal desktop.

agent/BACKLOG.md

@@ -28,15 +28,6 @@ in [`docs/ROADMAP.md`](../docs/ROADMAP.md); map in
 
 ## NEXT
 
-### 74. Unattended-install test matrix
-`test-install.sh` always LUKS+swap; add (or document) no-swap and
-explicit no-LUKS paths via env flags only. Cost: script + KVM time; V2.
-
-### 75. Installer template SoT — V2 install after ISO rebuild
-HM pre-activate now pulls starter packages (larger closure). Run
-`test-install.sh` (or equivalent) on a rebuilt live ISO and confirm
-first boot still themed. Cost: ISO + KVM; V2.
-
 ### 20. KVM runner → VM suite in CI `[human]`
 The remaining stretch of the CI item — checks-on-push is live and
 **green** (run #58; runner = gitea/act_runner docker, eval tier).