docs(runbook): use burnin-dummy repo in job/build examples

Switch the §3 and §4.3 examples to Simone/burnin-dummy (the canonical
burn-in test repo): `bash build.sh` (fake ~30s build) producing a `dist/`
artifact, so the examples exercise a real build + artifact collection end
to end.

docs/RUNBOOK-PhaseC.md

@@ -184,10 +184,11 @@ ci job \
   --job-id manual-$(date +%s) \
   --template-path /var/lib/ci/templates/LinuxBuild2404/LinuxBuild2404.vmx \
   --snapshot-name BaseClean-Linux \
-  --repo-url https://gitea.emulab.it/Simone/local-ci-cd-system.git \
+  --repo-url https://gitea.emulab.it/Simone/burnin-dummy.git \
   --branch main \
   --guest-os linux \
-  --build-command "echo build-ok" \
+  --build-command "bash build.sh" \
+  --guest-artifact-source dist \
   --use-git-clone
 ```
 
@@ -199,10 +200,11 @@ ci job \
   --job-id manual-$(date +%s) \
   --template-path /var/lib/ci/templates/LinuxBuild2404/LinuxBuild2404.vmx \
   --snapshot-name BaseClean-Linux \
-  --repo-url https://gitea.emulab.it/Simone/local-ci-cd-system.git \
+  --repo-url https://gitea.emulab.it/Simone/burnin-dummy.git \
   --branch main \
   --guest-os linux \
-  --build-command "echo build-ok" \
+  --build-command "bash build.sh" \
+  --guest-artifact-source dist \
   --use-git-clone \
   --gui
 ```
@@ -270,8 +272,8 @@ ci build run \
   --ip-address "$IP" \
   --guest-os linux \
   --ssh-user ci_build --ssh-key-path /var/lib/ci/keys/ci_linux \
-  --clone-url https://gitea.emulab.it/Simone/local-ci-cd-system.git --clone-branch main \
-  --build-command "echo build-ok" \
+  --clone-url https://gitea.emulab.it/Simone/burnin-dummy.git --clone-branch main \
+  --build-command "bash build.sh" \
   --guest-artifact-source dist
 ```