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"]

scripts/build-agent-image.mjs

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+#!/usr/bin/env node
+/**
+ * Build the Codeman agent base image locally (decision: "build locally on first
+ * use", see docs/docker-cases-plan.md). No registry account required.
+ *
+ * Usage:
+ *   node scripts/build-agent-image.mjs [--engine docker|podman] [--image <ref>] [--no-cache]
+ *
+ * Defaults: engine=docker (falls back to podman if docker is absent),
+ *           image=codeman/agent:base
+ */
+import { spawn, spawnSync } from 'node:child_process';
+import { fileURLToPath } from 'node:url';
+import { dirname, join } from 'node:path';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const REPO_ROOT = join(__dirname, '..');
+const DOCKERFILE = join(REPO_ROOT, 'docker', 'agent.Dockerfile');
+const DEFAULT_IMAGE = 'codeman/agent:base';
+
+function parseArgs(argv) {
+  const args = { image: DEFAULT_IMAGE, engine: undefined, noCache: false };
+  for (let i = 0; i < argv.length; i++) {
+    const a = argv[i];
+    if (a === '--image') args.image = argv[++i];
+    else if (a === '--engine') args.engine = argv[++i];
+    else if (a === '--no-cache') args.noCache = true;
+    else if (a === '-h' || a === '--help') args.help = true;
+  }
+  return args;
+}
+
+function engineAvailable(engine) {
+  const r = spawnSync(engine, ['--version'], { stdio: 'ignore' });
+  return r.status === 0;
+}
+
+function resolveEngine(preferred) {
+  if (preferred) {
+    if (!engineAvailable(preferred)) {
+      console.error(`[build-agent-image] engine "${preferred}" not found on PATH`);
+      process.exit(1);
+    }
+    return preferred;
+  }
+  if (engineAvailable('docker')) return 'docker';
+  if (engineAvailable('podman')) return 'podman';
+  console.error('[build-agent-image] neither docker nor podman found on PATH. Install one and retry.');
+  process.exit(1);
+}
+
+const args = parseArgs(process.argv.slice(2));
+if (args.help) {
+  console.log('Usage: node scripts/build-agent-image.mjs [--engine docker|podman] [--image <ref>] [--no-cache]');
+  process.exit(0);
+}
+
+const engine = resolveEngine(args.engine);
+const buildArgs = ['build', '-f', DOCKERFILE, '-t', args.image];
+if (args.noCache) buildArgs.push('--no-cache');
+buildArgs.push(REPO_ROOT);
+
+console.log(`[build-agent-image] ${engine} ${buildArgs.join(' ')}`);
+const child = spawn(engine, buildArgs, { stdio: 'inherit' });
+child.on('exit', (code) => {
+  if (code === 0) {
+    console.log(`\n[build-agent-image] built ${args.image}. Docker cases can now launch.`);
+  } else {
+    console.error(`\n[build-agent-image] build failed (exit ${code}).`);
+  }
+  process.exit(code ?? 1);
+});