docs: record architecture decisions (ADR-0001..0009) and operations knowledge

Add docs/adr/ with a chronological ADR set reconstructed from the project
history (no orchestrator, one storage format, derived-state sync,
SQL-over-SSH contract, WireGuard beneath SSH, IaC boundaries, non-root
image, runtime data paths, bounded lake scans), an index README, and a
template. Distinguish repo-local ADRs from ecosystem-wide ASRs.

Expand AGENTS.md with a decision-records section and a runtime/operations
guide (service URLs, rebuild workflow, CPU budget knobs). Reference the
ADRs from README, CONTRIBUTING, and the affected design docs, and add an
open question for T1 partition pruning.

docs/adr/ADR-0007-non-root-image.md

@@ -0,0 +1,27 @@
+# ADR-0007: Run the simulator image as a non-root user
+
+## Status
+
+Accepted (2026-07-08)
+
+## Context
+
+The Trivy scan in CI flagged **DS-0002**: the simulator image ran as `root`
+because no `USER` was set. A container writing the data lake as root also
+produces root-owned Parquet on bind mounts, which then blocks a later non-root
+process from the same files.
+
+## Decision
+
+Create a dedicated unprivileged user **`swarm` (uid/gid 10001)** in the
+`simulator/Dockerfile` and switch to it with `USER swarm` before the entrypoint.
+The uid is fixed (not auto-assigned) so host-side ownership of the shared data
+paths ([ADR-0008](ADR-0008-runtime-data-paths.md)) is deterministic.
+
+## Consequences
+
+- Trivy DS-0002 clears; the image follows least-privilege.
+- Bind-mounted data is owned by `10001:10001`; host prep (`sim-cluster.yml`) and
+  Ansible `chown` the `var/t1` / `var/t3` paths to this uid so k3d pods can write.
+- Any future service image reusing this data must run as the same uid or share
+  the group.