feat(delivery): docker sandbox adapter + owner-only infrastructure (v2 phase A3-1)

The execution boundary for v2. Adds the local docker/podman sandbox adapter
and the hardened host subprocess runner every other delivery adapter will use.

- adapters/process.py: the one place the platform shells out. argv-only (no
  shell), start_new_session + killpg on timeout, concurrent stdout/stderr
  drains with a byte cap, explicit minimal env (never os.environ), and a
  missing binary reported as spawn_failed rather than an exception.
- adapters/docker_sandbox.py: DockerSandboxAdapter over SandboxProviderPort.
  --user 1000:1000, --cap-drop ALL, no-new-privileges, pids/cpu/memory limits,
  --read-only with tmpfs scratch, one bind mount (the work tree, never the git
  dir), labels, no published ports. Path-safe file access; secrets refused in
  `docker exec -e`. Extras for later phases: start_demo_container (loopback
  publish only), list_managed_containers, remove_container.
- registry: SANDBOX_PROVIDER=docker, cached singleton so the workspace registry
  survives create_workspace -> start_session.
- sandbox_gateway worker: reap_orphans() removes labelled containers no live
  session points at, audited as sandbox.reap; run_pass() does both passes.
- sandbox router: every mutating route now requires the platform owner
  (PLAN-v2 §7.7); read-only status routes stay open to org members.
- sandbox-panel: canManageInfra prop (fails closed) hides the create form,
  the session panel and the action buttons from non-owners.

Tests: 346 API (+37), 67 web (+3); ruff, mypy, tsc and lint clean.
Three existing tests now drive the sandbox as the platform owner, since that
is who may touch it.

Report: docs/plan/reports/agent-3-phase-1.md

apps/web/src/app/(app)/projects/[id]/page.tsx

@@ -34,7 +34,7 @@ export default function ProjectDetailPage({
   params: Promise<{ id: string }>;
 }) {
   const { id: projectId } = React.use(params);
-  const { activeOrganization } = useSession();
+  const { activeOrganization, user } = useSession();
 
   const [tab, setTab] = React.useState<Tab>("repo");
   const [project, setProject] = React.useState<Project | null>(null);
@@ -136,7 +136,11 @@ export default function ProjectDetailPage({
       ) : tab === "preview" ? (
         <PreviewDomainPanel orgId={activeOrganization.id} projectId={project.id} />
       ) : (
-        <SandboxPanel orgId={activeOrganization.id} projectId={project.id} />
+        <SandboxPanel
+          orgId={activeOrganization.id}
+          projectId={project.id}
+          canManageInfra={user?.is_platform_owner ?? false}
+        />
       )}
     </div>
   );