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/07-observability.md

@@ -2,6 +2,8 @@
 
 The platform watches itself with the same discipline it applies to sensor data — and largely through the same pipeline.
 
+> **Decision:** [ADR-0009 — bound observability lake scans to a flight window and cache](adr/ADR-0009-bounded-lake-scans.md).
+
 ## Two kinds of signals, one storage
 
 | Signal | Examples | Where it goes |