feat(daemon): publishable Docker image + compose stack

[Codename-11]

Summary

Adds the ghcr.io/axiom-labs/arc-daemon Docker image + compose example for headless ARC installs. Runs arc-daemon --foreground on :7272 as a non-root user with state mounted at /home/arc/.arc.

• Multi-stage Dockerfile (packages/daemon/Dockerfile) — stage 1 installs a filtered pnpm subgraph (core, client, daemon), rebuilds better-sqlite3 against the container Node, runs tsup + pnpm deploy --prod. Stage 2 is node:20-bookworm-slim + tini + curl, non-root uid 1000, VOLUME /home/arc/.arc, HEALTHCHECK probing /health.
• Standalone CLI (packages/daemon/src/cli.ts) — tiny flag parser (--port/--host/--arc-dir) used as the ENTRYPOINT and registered as the arc-daemon bin. The richer arc daemon start remains canonical.
• tsup config (packages/daemon/tsup.config.ts) — daemon-scoped build that inlines workspace siblings and externalises real deps so Node can resolve CJS modules like ws + better-sqlite3 at runtime.
• docker-compose.yml — loopback-only daemon service with a watchtower sidecar behind the auto-update profile.
• DOCKER.md — operator guide (pull/run, port mapping, volume semantics, security notes).
• GitHub Actions workflow (.github/workflows/docker-daemon.yml) — pushes to GHCR on v* tags and on workflow_dispatch; matrixed over linux/amd64 + linux/arm64 with a manifest-merge job.
• Deps — daemon now explicitly declares @axiom-labs/arc-client and ws, which were previously leaking in as transitives.

Does NOT wire an external DB, change the default port (7272), or publish the image (CI does that on tag).

Test plan

• pnpm install — clean
• npx tsc --noEmit — passes
• pnpm --filter @axiom-labs/arc-daemon build — produces dist/cli.js + dist/index.js
• Standalone binary: node dist/cli.js --foreground --port 17272 --host 127.0.0.1 — daemon starts, curl /health returns {"ok":true,"protocol":1,...}, SIGTERM shuts down cleanly
• docker build -f packages/daemon/Dockerfile -t arc-daemon:test . — unverified locally: Docker Desktop's Linux engine isn't running in this worktree. CI workflow will exercise it on the first manual dispatch.
• docker run + /health round-trip — unverified locally (same reason)

Notes for reviewers

• The --foreground flag is accepted by the standalone bin for parity with arc daemon start, but the bin is always foreground (no daemonisation). That's documented in --help and in DOCKER.md.
• Inside the container ARC_HOST=0.0.0.0 binds all interfaces, but the daemon's existing Host-header allowlist still requires 127.0.0.1 / localhost. Publishing with -p 127.0.0.1:7272:7272 (the documented default) keeps access loopback-only on the host. LAN exposure requires a proxy that rewrites Host — called out in DOCKER.md.
• pnpm deploy --prod --legacy is used to produce a flat production tree without workspace symlinks. The --legacy flag is required on pnpm 10 because injected-workspace-packages conflicts with --prod pruning.