✨ feat(server): add go_version to /api/info response

Adds a "go_version" field to InfoResponse, populated via runtime.Version().
Useful for ops debugging (which Go version is running in production?).
BDD scenario @critical asserts the field is non-empty. Doc + ADR-0026
updated.

adr/0026-composite-info-endpoint.md

@@ -141,10 +141,13 @@ We will implement a new `GET /api/info` endpoint that returns a JSON object with
   "build_date": "2026-05-04T08:00:00Z",
   "uptime_seconds": 1234,
   "cache_enabled": true,
-  "healthz_status": "healthy"
+  "healthz_status": "healthy",
+  "go_version": "go1.26.1"
 }
 ```
 
+The `go_version` field provides the Go runtime version via `runtime.Version()`, useful for ops debugging (e.g., identifying which Go version is running in production).
+
 ### Rationale
 
 1. **Performance**: Single HTTP request instead of 3-4 separate calls