feat(example): persist the agent workspace on the PVC via subPath (#34)

/workspace - the DSH image's WORKDIR, where agents keep files - was plain
container storage, so every suspend/resume or pod replacement silently threw
agent work away while DSH state survived on the PVC.

Mount the same per-sandbox `dsh-home` PVC a second time at /workspace with
`subPath: workspace`. One PVC, no extra provisioning, and the subPath keeps
DSH state (sessions, config and .credentials.yaml at the volume root) out of
the agent-visible tree; mounting the volume root at /workspace would have
exposed the credential file.

The kubelet creates the missing subPath directory itself and chmods it to the
volume's fsGroup permissions, and the volume root's setgid bit makes the new
directory inherit gid 1000, so UID 1000 can write with no init container and
no root. The rare interrupted-kubelet case that leaves 0755 is documented with
a recovery procedure (kubernetes#142464) rather than papered over.

- e2e-kind: write a /workspace marker alongside the DSH_HOME marker, assert
  the mount is the dsh-home subPath, and verify both survive suspend/resume.
- Docs: README (overview, steps 4-6, troubleshooting row), architecture
  (PVC diagram, persistence, component table), main-image working-directory
  row, platform runbook PVC row.

Verified: make check (14 manifests), make lint, plus YAML assertions on the
two mounts and the unchanged volumeClaimTemplates.

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

examples/dsh-agents-as-a-service/manifests/10-sandboxtemplate.yaml

@@ -135,7 +135,8 @@ spec:
             periodSeconds: 15
             failureThreshold: 3
           # The container only needs to serve HTTP; drop every capability and
-          # disallow privilege escalation. DSH writes only to DSH_HOME (PVC).
+          # disallow privilege escalation. DSH writes only to its PVC-backed
+          # directories (DSH_HOME and the agent workspace).
           securityContext:
             allowPrivilegeEscalation: false
             capabilities:
@@ -148,6 +149,16 @@ spec:
           volumeMounts:
             - name: dsh-home
               mountPath: /home/node/.dsh # DSH_HOME: sessions, config, state
+            # The agent workspace (the image's WORKDIR) is PVC-backed too, but
+            # under its own subPath so DSH state - including .credentials.yaml -
+            # never shows up as workspace data. Kubelet creates workspace/ on
+            # first mount with the volume's fsGroup permissions (0775, setgid
+            # inherited from the volume root), so UID 1000 can write; no init
+            # container is needed. If the kubelet is killed mid-creation the
+            # directory stays 0755: see Troubleshooting in the README.
+            - name: dsh-home
+              mountPath: /workspace
+              subPath: workspace
   volumeClaimTemplates:
     - metadata:
         name: dsh-home