Phase 2: single per-user box container for every thread

Every thread (agent turns + terminal + project commands) now execs into one
persistent per-user container (spoon-box-{username}) instead of ephemeral
docker run --rm — so the agent and terminal share the exact same running
environment, filesystem, and in-session installs.

- docker.ts: ensureUserContainer (persistent box) + streamExecInContainer/
  runExecInContainer (docker exec, streaming) sharing a factored streamSubprocess
- user-container.ts: reference-counted box lifecycle (held while any thread
  workspace is active or a terminal is connected; idle-reaped after
  SPOON_AGENT_BOX_IDLE_MS, default 30m)
- worker.ts: runClaim acquires the box; codex turn + runProjectCommand exec into
  it; release on stop/PR/failure
- terminal.ts: execs into the shared box (dockerode TTY) instead of a per-job
  container; materializeUserHome runs the dotfiles setup in the box
- Verified: agent + terminal run in the same box, share fs, dotfiles + tmux load

apps/agent-worker/src/user-container.ts

@@ -0,0 +1,40 @@
+import { env } from './env';
+import {
+  ensureUserContainer,
+  stopWorkspaceContainer,
+  userContainerName,
+} from './runtime/docker';
+
+// Phase 2: one persistent "box" container per user that all of their threads
+// (agent turns + terminal + commands) exec into. Reference-counted so it stays
+// up while any thread workspace is active or a terminal is connected, and is
+// reaped after an idle period once nothing holds it.
+type Box = { refs: number; idleTimer?: NodeJS.Timeout };
+const boxes = new Map<string, Box>();
+
+export const acquireUserBox = async (args: {
+  username: string;
+  workdir: string;
+  containerHome: string;
+}): Promise<string> => {
+  const name = await ensureUserContainer(args);
+  const box = boxes.get(args.username) ?? { refs: 0 };
+  if (box.idleTimer) {
+    clearTimeout(box.idleTimer);
+    box.idleTimer = undefined;
+  }
+  box.refs += 1;
+  boxes.set(args.username, box);
+  return name;
+};
+
+export const releaseUserBox = (username: string) => {
+  const box = boxes.get(username);
+  if (!box) return;
+  box.refs = Math.max(0, box.refs - 1);
+  if (box.refs > 0) return;
+  box.idleTimer = setTimeout(() => {
+    void stopWorkspaceContainer(userContainerName(username));
+    boxes.delete(username);
+  }, env.boxIdleMs);
+};