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/VexHub/README.md

@@ -0,0 +1,17 @@
+# VexHub
+
+**Container(s):** stellaops-vexhub-web
+**Slot:** 11  |  **Port:** 8080  |  **Consumer Group:** vexhub
+**Resource Tier:** light
+
+## Purpose
+VexHub is the centralized VEX (Vulnerability Exploitability eXchange) document repository for the platform. It stores, indexes, and serves VEX statements from multiple sources with API-key-based authentication, supporting document search, filtering, and administrative operations.
+
+## API Surface
+- `vexhub` (via Router) — VEX document CRUD, search, filtering, provider management, admin operations
+
+## Storage
+PostgreSQL (via `Postgres:ConnectionString`, schema `vexhub`); Valkey for cache
+
+## Background Workers
+None