Add a startup probe so slow DSH cold starts are not killed (#6)

DSH takes tens of seconds to load plugins and start serving on a small
node, and every resume starts cold. The liveness budget (initialDelay
15s, period 15s, failureThreshold 3) is shorter than that, so the kubelet
killed the container mid-startup. DSH exits 0 on SIGTERM, which the
sandbox controller records as PodSucceeded/Finished - the pod is then
never recreated and the sandbox stays unusable until a manual
suspend/resume cycle.

Gate liveness and readiness behind a startupProbe on /favicon.svg with a
120s budget (24 x 5s, matching the gateway's WAKE_TIMEOUT). Liveness still
catches a genuinely hung server once startup succeeds.

Co-authored-by: SREDevOpsAgents <agents@sredevops.org>

deploy/k8s/sandbox-template.yaml

@@ -58,7 +58,15 @@ spec:
             - name: dsh-home
               mountPath: /home/node/.dsh # DSH_HOME
           # / is behind DSH's launch-token cookie (401 without it); use an
-          # unauthenticated path for probes.
+          # unauthenticated path for probes. The startup probe gives a cold
+          # start (plugin activation on a small node, or the first resume of a
+          # suspended sandbox) up to 120s before liveness can restart it.
+          startupProbe:
+            httpGet:
+              path: /favicon.svg
+              port: 3080
+            periodSeconds: 5
+            failureThreshold: 24
           readinessProbe:
             httpGet:
               path: /favicon.svg