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>

frontend/src/stores/systems.ts

@@ -39,7 +39,10 @@ export const useSystemsStore = defineStore("systems", () => {
     const list = systemsByProject.value[projectId];
     if (list) {
       const idx = list.findIndex((s) => s.id === systemId);
-      if (idx >= 0) list[idx] = system;
+      // Merged, not replaced: the PATCH reply is the bare System, and the
+      // list's read-side decorations (issue count, rulings usage) would
+      // otherwise vanish from the row until the next reload.
+      if (idx >= 0) list[idx] = { ...list[idx], ...system };
     }
     return system;
   }