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>

tests/test_retrieval_registry.py

@@ -41,6 +41,9 @@ SRC = Path(__file__).resolve().parents[1] / "src"
 RECORDERS = {
     "record_retrieval", "record_surfaced", "record_pulled",
     "record_rule_surfaced", "record_rule_pulled", "rules_payload",
+    # The rulings arm (#4769): its two recorders, and `rulings_for_paths`,
+    # which forwards its caller's `source` the way `rules_payload` does.
+    "record_system_surfaced", "record_system_pulled", "rulings_for_paths",
 }