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/TEMPLATE.md

@@ -0,0 +1,29 @@
+# ADR-NNNN: Short decision title
+
+## Status
+
+Proposed | Accepted | Superseded by ADR-XXXX
+
+## Context
+
+What forces are in play? What problem or constraint triggered the decision?
+Keep it to the facts that actually shaped the choice.
+
+## Options
+
+| Option | Pros | Cons |
+| --- | --- | --- |
+| A — … | … | … |
+| B — … | … | … |
+
+(Drop this section when only one option was ever real.)
+
+## Decision
+
+The choice, stated plainly. One paragraph.
+
+## Consequences
+
+- What becomes easier.
+- What becomes harder or is now forbidden.
+- Follow-ups, links to affected docs.