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.

CONTRIBUTING.md

@@ -81,9 +81,18 @@ identifying names of people, companies, or locations.
 - [ ] Tests added or updated; `pytest tests/ -q` passes locally
 - [ ] `docker build` in `simulator/` succeeds (test stage included)
 - [ ] Documentation updated if behaviour or boundaries changed
+- [ ] **ADR added** if the change alters an architectural boundary, contract, or trade-off (see [`docs/adr/`](docs/adr/))
 - [ ] English only; no identifying references
 - [ ] Conventional commit message
 
+## Decisions
+
+Architectural decisions are recorded chronologically as ADRs in
+[`docs/adr/`](docs/adr/). Read [`docs/adr/README.md`](docs/adr/README.md) before
+changing a boundary or contract; add a new ADR (from
+[`docs/adr/TEMPLATE.md`](docs/adr/TEMPLATE.md)) rather than editing an Accepted
+one when you reverse course.
+
 ## Questions
 
 Open design questions live in [`docs/09-open-questions.md`](docs/09-open-questions.md).