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/Signals/README.md

@@ -0,0 +1,17 @@
+# Signals
+
+**Container(s):** stellaops-signals
+**Slot:** 44  |  **Port:** 8080  |  **Consumer Group:** signals
+**Resource Tier:** light
+
+## Purpose
+The Signals service ingests and routes events from SCM webhooks (Gitea, GitHub, GitLab), container registries, and other sources into the Stella Ops platform. It parses webhook payloads, maps them to internal signal models, and forwards them to downstream services (Scanner, Scheduler) for processing. It also provides SCM integration for repository and commit queries.
+
+## API Surface
+- `signals` (via Router) — webhook ingestion endpoints, SCM repository queries, commit history, signal routing, event replay
+
+## Storage
+PostgreSQL (via `ConnectionStrings:Default`); Valkey for event routing
+
+## Background Workers
+None (event processing is synchronous on webhook receipt)