Document coding agents and give them a sandbox image

Agents work on this repository in the agent-envs sandbox: a container
whose only way out is an egress proxy with an allowlist, which also adds
the Gitea bot token. docs/contributing/agents.md covers setting it up
for Politrack (1Password item, .env, commit identity), what it can reach
and its limits; AGENTS.md tells agents how to recognise the sandbox and
behave in it.

backend/tests/Dockerfile.debian gains a `sandbox` target: the agent-envs
agent image plus the backend's dependencies from Debian packages, the
same list as the PyPI-free test image (still the default target). The
sandbox runs `pytest` without PyPI, which the owner's PC can't reach. A
new invariant in AGENTS.md keeps that list in step with pyproject.toml.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/tests/Dockerfile.debian

@@ -1,11 +1,24 @@
-# Test image with the Python dependencies from Debian packages, for networks where PyPI is unreachable.
-# Lacks procrastinate and mcp (not packaged in Debian), so it runs the test suite and adapters, not the
-# workers or the MCP server. Usage: docs/contributing/testing.md ("Without PyPI access").
-FROM debian:trixie-slim
-RUN apt-get update -qq && apt-get install -y -qq --no-install-recommends \
+# The Python dependencies from Debian packages, for networks where PyPI is unreachable. Lacks
+# procrastinate and mcp (not packaged in Debian), so it runs the test suite and adapters, not the
+# workers or the MCP server; ruff isn't packaged either.
+#
+# Two images share this package list; keep it in step with backend/pyproject.toml:
+# - the test image, the default target: docs/contributing/testing.md ("Without PyPI access");
+# - the agent sandbox image, target `sandbox` with BASE set to the agent-envs agent image, which
+#   agent-envs builds from this checkout: docs/contributing/agents.md.
+ARG BASE=debian:trixie-slim
+FROM ${BASE} AS deps
+# eatmydata skips dpkg's disk syncs, which crawl on some Docker Desktop disks.
+RUN apt-get update -qq && apt-get install -y -qq --no-install-recommends eatmydata >/dev/null \
+    && eatmydata apt-get install -y -qq --no-install-recommends \
         python3 python3-psycopg python3-psycopg-pool python3-pydantic-settings python3-fastapi python3-httpx \
         python3-protego python3-feedparser python3-bs4 python3-slugify python3-rapidfuzz python3-typer \
         python3-yaml python3-uvicorn python3-pytest python3-openpyxl ca-certificates >/dev/null \
     && rm -rf /var/lib/apt/lists/*
+
+# Keeps the agent image's user, entrypoint and /workspace as they are.
+FROM deps AS sandbox
+
+FROM deps AS test
 WORKDIR /app
 ENV PYTHONPATH=/app PYTHONUNBUFFERED=1 PYTHONDONTWRITEBYTECODE=1