feat: Docker quickstart with Compose, docs, and improved Dockerfile

Rewrites Dockerfile to use bookworm-slim base, installs Claude and Codex
CLIs, adds docker-compose.quickstart.yml for one-command setup, and adds
DOCKER.md with usage instructions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.quickstart.yml

@@ -0,0 +1,14 @@
+services:
+  paperclip:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    ports:
+      - "${PAPERCLIP_PORT:-3100}:3100"
+    environment:
+      HOST: "0.0.0.0"
+      PAPERCLIP_HOME: "/paperclip"
+      OPENAI_API_KEY: "${OPENAI_API_KEY:-}"
+      ANTHROPIC_API_KEY: "${ANTHROPIC_API_KEY:-}"
+    volumes:
+      - "${PAPERCLIP_DATA_DIR:-./data/docker-paperclip}:/paperclip"