fix(minimax,z-ai): stage api_key as root, restore mount mode 0400

Mirrors agent-runtimes commit 6f20b51. CRS serves these harness files to
dispatchers, so this repo must match.

Switches from "mount mode 0444 so the agent user can `cat` the ESO file"
to the gitea-ssh staging pattern: keep the ESO mount root-only (0400),
init.sh as root `install`s a per-secret 0600 agent-owned copy, and
apiKeyHelper points at the staged copy. Stronger blast-radius guarantee
— if the ESO Secret later grows additional keys, they remain root-only
unless the harness explicitly stages them.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

harnesses/contexts/z-ai/v1/harness.yaml

@@ -15,10 +15,12 @@ secrets_required:
   - name: z-ai
     account_ref: "z-ai"
     mount_path: /run/agent/secrets/z-ai
-    # 0444 — readable by the agent user that runs `claude` (and thus
-    # apiKeyHelper). Pod-local tmpfs; "world readable" only means readable
-    # by processes in this pod.
-    mode: "0444"
+    # 0400 (root-only) — defense in depth. The agent user cannot read this
+    # mount. init.sh runs as root and stages the auth_token into the agent's
+    # home with mode 0600. apiKeyHelper points at the staged copy. Matches
+    # the gitea-ssh pattern; if the ESO Secret later carries additional
+    # files (e.g. base_url is already there), they remain inaccessible.
+    mode: "0400"
 
 scripts:
   init: "./init.sh"