docs: add service README.md files + update AGENTS.md decisions

- Create README.md for 25+ service modules with container info, API surface, storage
- Document attestor-tileproxy separation rationale (air-gap network isolation)
- Document opsmemory-advisoryai separation rationale (resource isolation, blast radius)
- Update Timeline AGENTS.md with merged indexer info

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ReachGraph/README.md

@@ -0,0 +1,17 @@
+# ReachGraph
+
+**Container(s):** stellaops-reachgraph-web
+**Slot:** 22  |  **Port:** 8080  |  **Consumer Group:** reachgraph
+**Resource Tier:** light
+
+## Purpose
+The ReachGraph service provides content-addressed storage and retrieval of reachability subgraphs. It stores canonical serialized reachability data with digest-based addressing, uses PostgreSQL for persistence and Valkey for caching, and supports rate-limited access for high-throughput graph queries.
+
+## API Surface
+- `reachgraph` (via Router) — reachability subgraph storage (PUT), retrieval by digest (GET), comparison, rate-limited queries
+
+## Storage
+PostgreSQL (via `ConnectionStrings:PostgreSQL`); Valkey cache (via `ConnectionStrings:Redis`)
+
+## Background Workers
+None