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>

docs/contributing/testing.md

@@ -26,10 +26,14 @@ docker build -t politrack-test -f backend/tests/Dockerfile.debian backend/tests
 docker run --rm -v "$PWD/backend:/app" politrack-test python3 -m pytest -q -p no:cacheprovider
 ```
 
-The same image can run adapters against the compose database: add `--network politics-tracking_default`
-and `-e POLITRACK_DATABASE_URL=postgresql://politrack:<password>@db:5432/politrack`, then
+The test image can also run adapters against the compose database: add
+`--network politics-tracking_default` and
+`-e POLITRACK_DATABASE_URL=postgresql://politrack:<password>@db:5432/politrack`, then
 `python3 -m politrack.cli run <adapter> --max-requests 20`.
 
+The same package list builds the agent sandbox image (target `sandbox`, see
+[agents.md](agents.md#the-sandbox-image)), so keep it in step with `pyproject.toml`.
+
 ## Golden fixtures
 
 ```