Add AGENTS.md for agents picking up work in new sessions

- AGENTS.md: ground rules (legal access, evidence, fairness), invariants
  that are easy to break, commands, and notes on the owner's dev machine;
  CLAUDE.md imports it
- backend/tests/Dockerfile.debian: test image from Debian packages for
  networks where PyPI is unreachable, documented in testing.md
- roadmap: list what v2 has not been verified against yet

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

backend/tests/Dockerfile.debian

@@ -0,0 +1,11 @@
+# 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 \
+        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/*
+WORKDIR /app
+ENV PYTHONPATH=/app PYTHONUNBUFFERED=1 PYTHONDONTWRITEBYTECODE=1