feat(agent): real nix (sandbox=false) for firmware SBOM, replacing nix-portable

nix-portable fell back to proot in the deployment (user namespaces are blocked
by the container's default seccomp/apparmor profile, and orca can't relax it),
and proot corrupts the nix build's file-permission syscalls — every firmware
build failed at `cp: setting permissions … No such file or directory` and fell
back to the analysis-only SBOM.

Ship a real nix instead and disable its build sandbox (`sandbox = false`): a
plain gcc/make firmware build needs no user namespace, so it runs under the
locked-down profile with no proot at all. The store ships as a compressed
bootstrap tarball (built in a throwaway `nixos/nix` stage) and is seeded onto
/nix at first start by docker/agent-entrypoint.sh, so a persistent /nix volume
survives redeploys. Seeding and the whole path are best-effort — a broken nix
just falls back to analysis-only, never breaking a scan.

No agent code change: NixBackend::detect() already prefers the system `nix`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker/agent-entrypoint.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+# Seed the nix store on first start, then run the agent.
+#
+# The firmware-SBOM pipeline drives a real `nix` build (tramiton NixBackend).
+# The image ships the store as a bootstrap tarball rather than baking /nix, so a
+# persistent /nix volume (mounted empty on first deploy) gets populated once and
+# then survives redeploys. Seeding is best-effort: if it fails, the agent still
+# starts and firmware SBOMs fall back to analysis-only.
+if [ ! -e /nix/store ]; then
+  echo "agent-entrypoint: seeding /nix store from image bootstrap..."
+  mkdir -p /nix
+  if tar -C / -xzf /opt/nix-bootstrap.tar.gz; then
+    echo "agent-entrypoint: /nix store seeded."
+  else
+    echo "agent-entrypoint: WARN nix seed failed; firmware SBOM will use analysis-only fallback."
+  fi
+fi
+
+exec compliance-agent "$@"