feat(rulings): system_usage_events is read back — per-System counts and a telemetry block (#4769)

#4769 "Rulings are counted where someone will read them": milestone 444
step 4 wrote system_usage_events and nothing read it.

- retrieval_telemetry gains a `system_usage` block: surfacings and opens by
  source, distinct counts, and `by_system` naming the areas most shown.
  There is deliberately no pull-through ratio, because rulings travel in full
  in the line and opens are the exception.
- usage_for_systems (one GROUP BY) adds `usage` to the REST Systems list and
  detail, and to MCP get_system. MCP list_systems is unchanged.
- The Systems UI shows a "rulings shown N×" chip.
- rulings_pre_tool, rulings_write_path and mcp_get_system are now declared
  registry points; the registry guard covers their recorders.
- The Systems store merges a PATCH reply instead of replacing the row.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/routes/systems.py

@@ -10,6 +10,7 @@ from scribe.routes.utils import not_found
 from scribe.services import systems as systems_svc
 from scribe.services.access import can_write_project
 from scribe.services.projects import get_project_for_user
+from scribe.services import system_usage as system_usage_svc
 
 logger = logging.getLogger(__name__)
 
@@ -43,10 +44,14 @@ async def list_systems_route(project_id: int):
         uid, project_id, include_archived=_truthy(request.args.get("include_archived")),
     )
     counts = await systems_svc.open_issue_counts_by_system(uid, project_id)
+    # How often each area's rulings reached a session (#4769) — one GROUP BY
+    # for the page, and a zero shape for an area nothing has touched.
+    usage = await system_usage_svc.usage_for_systems([s.id for s in systems])
     out = []
     for s in systems:
         d = s.to_dict()
         d["open_issue_count"] = counts.get(s.id, 0)
+        d["usage"] = usage.get(s.id, system_usage_svc.empty_system_usage())
         out.append(d)
     return jsonify({"systems": out})
 
@@ -114,6 +119,7 @@ async def get_system_route(project_id: int, system_id: int):
     )
     data = system.to_dict()
     data["issues"], data["tasks"], data["notes"] = issues, tasks, notes
+    data["usage"] = (await system_usage_svc.usage_for_systems([system.id]))[system.id]
     return jsonify(data)