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/api/systems.ts

@@ -1,5 +1,6 @@
 import { apiGet, apiPost, apiPatch, apiDelete } from "@/api/client";
 import type { CanonicalMatch } from "@/api/canonicalSystems";
+import type { RecordUsage } from "@/types/usage";
 
 export interface System {
   id: number;
@@ -21,6 +22,13 @@ export interface System {
    */
   path_patterns: string[];
   open_issue_count: number;
+  /**
+   * How often this area's rulings were shown to a session because its files
+   * were touched (`surfaced_count`), and how often it was opened (#4769).
+   * Present on the list and detail reads; optional because a System returned
+   * by a create or update carries none.
+   */
+  usage?: RecordUsage;
   created_at: string | null;
   updated_at: string | null;
 }