Terminal: job image tools (neovim/tmux), build-arg wiring, docs

- agent-job image: add neovim, tmux, less, unzip, wget, locales for the
  interactive shell (tmux powers cross-reconnect session persistence)
- Wire NEXT_PUBLIC_SPOON_AGENT_WORKER_WS_URL as a build arg (Dockerfile +
  compose.yml) since NEXT_PUBLIC vars are inlined at build time
- docs/agent-terminal.md: architecture, env, nginx WS exposure, dev testing,
  security; note the build-time var in docs/compose.prod.yml

docker/agent-job.Dockerfile

@@ -11,9 +11,15 @@ RUN apt-get update \
     curl \
     git \
     jq \
+    less \
+    locales \
+    neovim \
     openssh-client \
     python3 \
     ripgrep \
+    tmux \
+    unzip \
+    wget \
   && corepack enable \
   && corepack prepare pnpm@latest --activate \
   && corepack prepare yarn@stable --activate \