feat: include only dsh-context in agent VM setup

Install through the upstream CLI before Web startup, preserve existing profile state and skip completed installs. Keep pnpm's SQLite store on the guest cache disk and map the caller to namespace root for 9p atomic saves, without host-root privileges.

Cover context-only installation, retries, idempotence and preservation with offline checks. Validate real VM boot, ports, Firefox and a live DSH/context launch using a disposable profile.

agent-vm/fake-dsh.sh

@@ -0,0 +1,29 @@
+# Offline CLI fixture only: no npm, network installs, credentials or model calls.
+set -euo pipefail
+profile="$DSH_HOME/profiles/web"
+manifest="$profile/package.json"
+if [[ ${1:-} == plugin ]]; then
+  [[ $# == 5 && $2 == --profile && $3 == web && $4 == add && $5 == dsh-context@* ]]
+  printf '%s\n' "$*" >> "$DSH_HOME/plugin-calls"
+  mkdir -p "$profile"
+  if [[ ! -e $manifest ]]; then
+    printf '%s\n' '{"dsh":{"profile":{"bundles":["@deepseek-ai/dsh-base","@deepseek-ai/dsh-web-app"]}}}' > "$manifest"
+  fi
+  jq --arg spec "${5#dsh-context@}" '.dependencies["dsh-context"] = $spec' "$manifest" > "$manifest.tmp"
+  mv "$manifest.tmp" "$manifest"
+  # Simulate a failed install after pnpm has already recorded the dependency.
+  [[ ! -e $DSH_HOME/fail-plugin-install ]] || exit 42
+  mkdir -p "$profile/node_modules/dsh-context"
+  printf '%s\n' '{"name":"dsh-context","version":"0.0.0"}' > "$profile/node_modules/dsh-context/package.json"
+  jq '.dsh.profile.bundles |= (. + ["dsh-context"] | unique)' "$manifest" > "$manifest.tmp"
+  mv "$manifest.tmp" "$manifest"
+  exit 0
+fi
+[[ ${1:-} == web ]]
+# The service must finish plugin setup before starting the Web listener.
+[[ -f $profile/node_modules/dsh-context/package.json ]]
+jq -e '.dsh.profile.bundles | index("dsh-context") != null' "$manifest" >/dev/null
+exec node -e '
+  require("node:http").createServer((req, res) => res.end("agent-vm-test"))
+    .listen(3080, "127.0.0.1", () => console.log("http://127.0.0.1:3080/?token=offline-test"));
+'