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-0008-runtime-data-paths.md

@@ -0,0 +1,40 @@
+# ADR-0008: Shared `var/t1` / `var/t3` runtime paths; state out of Git
+
+## Status
+
+Accepted (2026-07-08)
+
+## Context
+
+Compose, the k3d fleet, and the ground offload job all need to read and write
+the same data lake, so that "go live" in the prototype shows the data the
+simulator just generated. Early on, Terraform state and the provider cache were
+accidentally committed (~6 MB of `*.tfstate` and a vendored provider binary),
+polluting history and risking stale/secret data in Git.
+
+## Decision
+
+Standardise two **gitignored** runtime directories at the repo root:
+
+| Path | Tier | Contents |
+| --- | --- | --- |
+| `var/t1/` | T1 live lake | In-flight Parquet from Compose or the k3d fleet |
+| `var/t3/` | T3 warehouse | Offloaded historical partitions |
+
+They are created automatically and overridable via
+`SWARM_T1_DIR` / `SWARM_WAREHOUSE_DATA` / `SWARM_SIM_DATA`. Treat `var/` as
+**runtime-only** per the ecosystem canonical layout. `.gitignore` excludes
+`*.tfstate`, `*.tfstate.backup`, and `.terraform/` while **keeping**
+`.terraform.lock.hcl` (the lock is source, the state is not).
+
+## Consequences
+
+- One lake feeds every component; the live prototype reflects real generated
+  data.
+- Git history carries no machine state or large binaries; `terraform` treats
+  its state as local/remote-backend concern, not versioned source.
+- Ownership of these paths is fixed to uid 10001
+  ([ADR-0007](ADR-0007-non-root-image.md)).
+- Recovery from earlier state drift is documented in
+  [`../../infra/terraform/README.md`](../../infra/terraform/README.md)
+  (`terraform import`).