Harden the Jupyter sandbox runner against env-secret exposure

Run each kernel under a scrubbed environment so untrusted code can never read
the host's secrets. A custom 'docsgpt-python' kernelspec launches ipykernel
through a wrapper that keeps only what the kernel needs (PATH, HOME, LANG, and
the Jupyter runtime/data dirs), dropping API keys, tokens, the database URL, and
the gateway token. The app selects this kernel by name via SANDBOX_KERNEL_NAME,
so the distinct name is never shadowed by the stock python3 spec. Per-session
workspaces are created mode 0700 (defense in depth under the shared uid). The
README documents the runner as a single trust domain and points to the Daytona
backend for per-tenant isolation.

deployment/sandbox/kernels/docsgpt-python/kernel.json

@@ -0,0 +1,12 @@
+{
+  "argv": [
+    "/opt/docsgpt/kernel-launch.sh",
+    "-f",
+    "{connection_file}"
+  ],
+  "display_name": "Python 3 (docsgpt-sandbox, scrubbed env)",
+  "language": "python",
+  "metadata": {
+    "debugger": true
+  }
+}