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/services/retrieval_registry.py

@@ -151,6 +151,23 @@ POINTS: dict[str, Point] = dict([
        "the fixed question asked when a task finishes: how should this report read",
        fixed_query=True),
 
+    # The rulings arm (milestone 444). A LOOKUP, not a ranker: a path falls
+    # under a System's patterns or it does not, so nothing here writes to
+    # retrieval_logs and no score-shaped warning can apply. Its rows are in
+    # `system_usage_events`, read back as `retrieval_telemetry`'s
+    # `system_usage` block (#4769).
+    _p("rulings_pre_tool", UNBIDDEN,
+       "an area's rulings, shown because a command touched its files",
+       expects_traffic=False,
+       quiet_because="speaks only once a System has both path_patterns and a "
+                     "Rulings section; an install where none does is "
+                     "correctly silent here"),
+    _p("rulings_write_path", UNBIDDEN,
+       "an area's rulings, shown because an edit touched its files",
+       expects_traffic=False,
+       quiet_because="same as rulings_pre_tool — no System with patterns "
+                     "and rulings, nothing to show"),
+
     # ── Asked: a caller wanted a ranked list ─────────────────────────────
     _p("mcp_search", ASKED, "an agent called search"),
     _p("wide_net", ASKED, "an agent asked what might apply, with no bar"),
@@ -186,6 +203,10 @@ POINTS: dict[str, Point] = dict([
     _p("mcp_get_process", PULL, "an agent opened a process"),
     _p("mcp_get_lesson", PULL, "an agent opened a lesson"),
     _p("mcp_get_rule", PULL, "an agent opened a rule"),
+    _p("mcp_get_system", PULL, "an agent opened a System", expects_traffic=False,
+       quiet_because="a System's rulings arrive in full in the rulings line, "
+                     "so opening one is the exception rather than the "
+                     "measure; a window without it is not a gap"),
     _p("rest_note", PULL, "a person opened a note", expects_traffic=False,
        quiet_because="web UI only"),
     _p("rest_task", PULL, "a person opened a task", expects_traffic=False,
@@ -220,6 +241,9 @@ FAN_OUT_SITES: dict[str, tuple[str, ...]] = {
     "scribe/services/plugin_context.py::record_surfaced(source=arm)": (
         "write_path_place", "write_path_semantic", "write_path_sync",
     ),
+    "scribe/services/system_rulings.py::record_system_surfaced(source=source)": (
+        "rulings_pre_tool", "rulings_write_path",
+    ),
     "scribe/services/rulebooks.py::record_rule_surfaced(source=source)": (
         "enter_project", "start_planning", "get_task", "get_project",
         "get_milestone",