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/06-environments.md

@@ -2,6 +2,8 @@
 
 Three infrastructures, one data contract. The layout, schemas, and pipelines are identical everywhere; only scale and lifetime differ.
 
+> **Decisions:** [ADR-0006 — IaC boundaries](adr/ADR-0006-iac-boundaries.md), [ADR-0008 — runtime data paths](adr/ADR-0008-runtime-data-paths.md).
+
 ```mermaid
 graph LR
     subgraph dev [3 — Dev and simulation]