feat(docker): agent base image + local build script

docker/agent.Dockerfile: node:22 + claude/codex/gemini/opencode CLIs + git/
tmux/ripgrep/curl, secret-free, OpenShift arbitrary-uid-writable HOME (gid 0).
scripts/build-agent-image.mjs: local build (decision "build locally on first
use"), docker/podman auto-detect, --engine/--image/--no-cache flags.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker/agent.Dockerfile

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+# Codeman agent base image (built locally by scripts/build-agent-image.mjs).
+#
+# Contains the agent toolchain (node + the CLIs + git/tmux/ripgrep) but NO
+# secrets: credentials are delivered at RUNTIME via bind mounts (~/.claude etc.)
+# or name-only `docker exec --env`, never baked in, so `docker save` exports stay
+# secret-free. tmux is a HARD prerequisite (the in-container tmux is what makes a
+# reconnect durable), so it is installed here and probed before launch.
+#
+# HOME is made writable by an ARBITRARY host uid via the OpenShift "gid 0,
+# group-writable" convention: on Linux we run `--user <hostUid>:0`, so the agent
+# uid is the host uid (workspace files stay host-owned) while gid 0 keeps $HOME
+# writable even though the uid is not the baked 1000.
+FROM node:22-bookworm-slim
+
+# Base toolchain. `curl` is needed for the hook callbacks (`curl -sk $CODEMAN_API_URL`),
+# `procps` for `ps`, `tmux` for the durable in-container session.
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends \
+      git \
+      tmux \
+      ripgrep \
+      curl \
+      ca-certificates \
+      less \
+      procps \
+      openssh-client \
+ && rm -rf /var/lib/apt/lists/*
+
+# The agent CLIs (all four backends Codeman supports). Pinning is left to the
+# rebuild cadence (see docs/docker-cases-plan.md, user-decision 2).
+RUN npm install -g \
+      @anthropic-ai/claude-code \
+      @openai/codex \
+      @google/gemini-cli \
+      opencode-ai \
+ && npm cache clean --force
+
+# `agent` user (uid 1000, gid 0) with an arbitrary-uid-writable HOME.
+ENV HOME=/home/agent
+RUN useradd -u 1000 -g 0 -m -d /home/agent -s /bin/bash agent \
+ && mkdir -p /home/agent/.npm /home/agent/.cache /home/agent/.config /home/agent/.codeman \
+ && chgrp -R 0 /home/agent \
+ && chmod -R g=u /home/agent
+
+USER 1000:0
+WORKDIR /home/agent
+
+# Codeman overrides the command with `sleep infinity` at create time; this is the
+# fallback so a hand-run container also idles rather than exiting.
+CMD ["sleep", "infinity"]