review: remove security-boundary note, move podman into nested-containers layer

- Drop the module-level paragraph emphasizing the single-UID security
  boundary (#issuecomment-5010): redundant given the safe environment.
- Move `podman` from each agent image (claude, codex, pi) into the
  `build_image` derived layer so bottles without `nested_containers`
  pay no image-size cost (#issuecomment-5013).
- Update `build_image` docstring and inline comments to reflect the move.
- Add TODO(#394) noting the planned docker-layer abstraction refactor.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/contrib/claude/Dockerfile

@@ -26,7 +26,6 @@ RUN apt-get update \
   ca-certificates \
   curl \
   openssh-client \
-  podman \
   ripgrep \
   iproute2 \
   dnsutils \

bot_bottle/contrib/codex/Dockerfile

@@ -11,7 +11,6 @@ RUN apt-get update \
   ca-certificates \
   curl \
   openssh-client \
-  podman \
   procps \
   ripgrep \
   && rm -rf /var/lib/apt/lists/*

bot_bottle/contrib/pi/Dockerfile

@@ -11,7 +11,6 @@ RUN apt-get update \
   curl \
   fd-find \
   openssh-client \
-  podman \
   ripgrep \
   && ln -s /usr/bin/fdfind /usr/local/bin/fd \
   && rm -rf /var/lib/apt/lists/*