fix(worker): keep-id compat, zombie tmux clients, nvm in box image

- Box init + chownInBox exec explicitly as root: under podman keep-id the
  container's default user is the mapped uid, which cannot useradd/chown.
- Init renames an existing uid-1000 passwd entry (podman keep-id injects one
  named after the HOST user) instead of adding a duplicate that would win the
  getpwuid lookup and make whoami report the wrong name.
- Terminal connections tag their shell with a per-connection marker and reap
  the process group on WebSocket close: closing the attach socket never
  killed the exec'd shell, so every disconnect leaked an attached tmux client
  (session stuck at the smallest zombie's size; enough dead clients starve
  tmux rendering for live ones).
- Job image: nvm 0.39.5 at /usr/local/nvm (user-writable) with
  /etc/profile.d/nvm.sh, matching what user dotfiles expect; docs for the box
  user + ownership model.

docker/agent-job.Dockerfile

@@ -50,6 +50,16 @@ RUN dnf install -y --setopt=install_weak_deps=False --nodocs \
 RUN npm install -g pnpm yarn bun@1.3.10 opencode-ai@1.17.9 @openai/codex@0.142.0 @anthropic-ai/claude-code@2.1.207 \
   && npm cache clean --force
 
+# nvm, system-wide: user dotfiles source /etc/profile.d/nvm.sh (workstation
+# parity), and NVM_DIR must be writable by the box user (uid 1000) so
+# `nvm install` works from the shell. PROFILE=/dev/null stops the installer
+# from editing root's rc files.
+ENV NVM_DIR=/usr/local/nvm
+RUN mkdir -p "$NVM_DIR" \
+  && wget -qO- https://raw.githubusercontent.com/nvm-sh/nvm/v0.39.5/install.sh | PROFILE=/dev/null bash \
+  && printf 'export NVM_DIR=/usr/local/nvm\n[ -s "$NVM_DIR/nvm.sh" ] && . "$NVM_DIR/nvm.sh"\n[ -s "$NVM_DIR/bash_completion" ] && . "$NVM_DIR/bash_completion"\n' > /etc/profile.d/nvm.sh \
+  && chown -R 1000:1000 "$NVM_DIR"
+
 # oh-my-posh prompt (binary only; we ship our own /etc/spoon/omp.json theme).
 RUN curl -fsSL https://ohmyposh.dev/install.sh | bash -s -- -d /usr/local/bin \
   && oh-my-posh version