feat(v2): builder-agent self-modification WIP + container-config as per-group file

Checkpoints the builder-agent dev-agent/worktree/swap flow (create_dev_agent,
request_swap, classifier, deadman, promote) before pivoting to a unified
draft-activate approach with OS-level RO enforcement. Lifts container_config
out of the agent_groups row into groups/<folder>/container.json so install_packages,
add_mcp_server, and rebuild flows can eventually route through the same draft
path as source edits.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/container-config.ts

@@ -0,0 +1,117 @@
+/**
+ * Per-group container config, stored as a plain JSON file at
+ * `groups/<folder>/container.json`. Replaces the former
+ * `agent_groups.container_config` DB column.
+ *
+ * Shape:
+ *   {
+ *     mcpServers:      { [name]: { command, args, env } }
+ *     packages:        { apt: string[], npm: string[] }
+ *     imageTag?:       string                    // set by buildAgentGroupImage on rebuild
+ *     additionalMounts?: Array<{hostPath, containerPath, readonly}>
+ *   }
+ *
+ * All fields are optional — a missing file or a partial file both resolve
+ * to sensible defaults. Writes are atomic-enough (write-then-rename is not
+ * worth the ceremony here since there's only one writer in practice: the
+ * host, from the delivery thread that processes approved system actions).
+ */
+import fs from 'fs';
+import path from 'path';
+
+import { GROUPS_DIR } from './config.js';
+
+export interface McpServerConfig {
+  command: string;
+  args?: string[];
+  env?: Record<string, string>;
+}
+
+export interface AdditionalMountConfig {
+  hostPath: string;
+  containerPath: string;
+  readonly?: boolean;
+}
+
+export interface ContainerConfig {
+  mcpServers: Record<string, McpServerConfig>;
+  packages: { apt: string[]; npm: string[] };
+  imageTag?: string;
+  additionalMounts: AdditionalMountConfig[];
+}
+
+function emptyConfig(): ContainerConfig {
+  return {
+    mcpServers: {},
+    packages: { apt: [], npm: [] },
+    additionalMounts: [],
+  };
+}
+
+function configPath(folder: string): string {
+  return path.join(GROUPS_DIR, folder, 'container.json');
+}
+
+/**
+ * Read the container config for a group, returning sensible defaults for
+ * any missing fields (or an entirely empty config if the file is absent).
+ * Never throws for missing / malformed files — corruption logs a warning
+ * via console.error and falls back to empty.
+ */
+export function readContainerConfig(folder: string): ContainerConfig {
+  const p = configPath(folder);
+  if (!fs.existsSync(p)) return emptyConfig();
+  try {
+    const raw = JSON.parse(fs.readFileSync(p, 'utf8')) as Partial<ContainerConfig>;
+    return {
+      mcpServers: raw.mcpServers ?? {},
+      packages: {
+        apt: raw.packages?.apt ?? [],
+        npm: raw.packages?.npm ?? [],
+      },
+      imageTag: raw.imageTag,
+      additionalMounts: raw.additionalMounts ?? [],
+    };
+  } catch (err) {
+    console.error(`[container-config] failed to parse ${p}: ${String(err)}`);
+    return emptyConfig();
+  }
+}
+
+/**
+ * Write the container config for a group, creating the groups/<folder>/
+ * directory if necessary. Pretty-printed JSON so diffs in the activation
+ * flow are reviewable.
+ */
+export function writeContainerConfig(folder: string, config: ContainerConfig): void {
+  const p = configPath(folder);
+  const dir = path.dirname(p);
+  if (!fs.existsSync(dir)) fs.mkdirSync(dir, { recursive: true });
+  fs.writeFileSync(p, JSON.stringify(config, null, 2) + '\n');
+}
+
+/**
+ * Apply a mutator function to a group's container config and persist the
+ * result. Convenient for append-style changes like `install_packages` and
+ * `add_mcp_server` handlers.
+ */
+export function updateContainerConfig(
+  folder: string,
+  mutate: (config: ContainerConfig) => void,
+): ContainerConfig {
+  const config = readContainerConfig(folder);
+  mutate(config);
+  writeContainerConfig(folder, config);
+  return config;
+}
+
+/**
+ * Initialize an empty container.json for a group if one doesn't already
+ * exist. Idempotent — used from `group-init.ts`.
+ */
+export function initContainerConfig(folder: string): boolean {
+  const p = configPath(folder);
+  if (fs.existsSync(p)) return false;
+  writeContainerConfig(folder, emptyConfig());
+  return true;
+}