ci: stop a job whose sandbox leaks, first thing

Installing the runner on sahaquiel, `useradd --system` gave `runner` user ID 999: the ID
under which PostgreSQL and the server run inside their containers, which Docker doesn't
remap. To the kernel, those processes were the runner's own, so a job could have read the
server's environment (database password, session pepper, GitHub secret) and, through
/proc, PostgreSQL's files and its local socket, which the official image trusts. The
sandbox's other walls don't help there: ProtectProc hides only other users' processes,
IPAddressDeny doesn't cover Unix sockets. Only our own check ran as 999; `runner` now has
user and group ID 61000.

The check job now starts with a Sandbox step that fails unless only the runner's own
processes are visible, all in one process namespace, /home/deploy is invisible, the
Docker socket is unusable and Docker's bridge network unreachable. It would have caught
this; outside a sandbox it fails as it should. The unit and AGENTS.md say how to create
the user.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

AGENTS.md

@@ -106,7 +106,7 @@ A spaced-repetition learning app: studysets of flashcards, reviewed on an FSRS s
 
 ## CI and deploys
 
-- **CI** (`.forgejo/workflows/ci.yaml`) runs `tool/check.sh` on every push, on the Forgejo runner on sahaquiel (`deploy/runner/`). Jobs run directly on the host as the unprivileged user `runner`, never in containers: Docker access would be root on the production server. The runner's systemd unit keeps jobs away from production (no `/home`, no Docker socket, Docker's container networks unreachable, memory and CPU capped); keep it that way when changing it. Flutter is pinned in the workflow (`FLUTTER_VERSION`): move it together with the local one. CI runs on pushes only: a pull request from a fork would run its code on the production server.
+- **CI** (`.forgejo/workflows/ci.yaml`) runs `tool/check.sh` on every push, on the Forgejo runner on sahaquiel (`deploy/runner/`). Jobs run directly on the host as the unprivileged user `runner`, never in containers: Docker access would be root on the production server. The runner's systemd unit keeps jobs away from production (no `/home`, no Docker socket, Docker's container networks unreachable, memory and CPU capped); keep it that way when changing it. `runner` has user ID 61000 and must never share one with a container's processes (PostgreSQL and the server run as 999, not remapped): the same user ID can read their environment and files. The job's first step (*Sandbox*) fails if any of this leaks. Flutter is pinned in the workflow (`FLUTTER_VERSION`): move it together with the local one. CI runs on pushes only: a pull request from a fork would run its code on the production server.
 - **A green `main` deploys learn.halkyon.eu** through `deploy/redeploy.sh`, the only command the runner's key may run as `deploy` (pinned in `authorized_keys`, from sahaquiel itself only). It deploys the commit CI checked while it is still the tip, as a fast-forward, only if every new commit is signed by a key in `deploy/allowed-signers` (the deployed copy decides), after dumping the database (the last 10 dumps stay in `~deploy/backups`), and fails unless the server answers. A failed deploy leaves the checkout at the deployed commit; run the workflow again to retry. By hand: `ssh sahaquiel-deploy halkyon-learn/deploy/redeploy.sh <commit>`. Never `docker compose down -v` there.
 
 ## Rules the code relies on