fix: use system libvirt with /tmp storage for virt-manager visibility

- Changed from qemu:///session to qemu:///system so VMs appear in virt-manager
- Store disk and ISO in /tmp (user-writable, no sudo needed)
- User is in libvirt group so can access system libvirt without sudo
- Updated test to expect system URI

This fixes the regression where VMs were not visible in virt-manager.

💘 Generated with Crush

Assisted-by: GLM-4.7 via Crush <crush@charm.land>

tests/unit/run_comprehensive_test.bats

@@ -253,8 +253,8 @@
     grep -q 'VM_DISK_SIZE=' /workspace/run.sh
 }
 
-@test "run.sh uses session libvirt URI (no root required)" {
-    grep -q 'qemu:///session' /workspace/run.sh
+@test "run.sh uses system libvirt URI" {
+    grep -q 'qemu:///system' /workspace/run.sh
 }
 
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