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/conftest.py

@@ -131,6 +131,26 @@ def _no_rulings_arm():
         yield
 
 
+@pytest.fixture(autouse=True)
+def _no_system_usage_readout():
+    """Stub the per-System usage lookup the Systems list and get_system carry
+    (#4769).
+
+    Autouse for _no_rulings_arm's reason: every route and tool test that
+    reads a System now reaches a GROUP BY on system_usage_events. Stubbed to
+    the zero shape for whatever ids were asked. tests/test_system_usage_readout.py
+    binds the real function at import time, before this patch runs.
+    """
+    from scribe.services.system_usage import empty_system_usage
+
+    async def _zeros(system_ids):
+        return {int(s): empty_system_usage() for s in system_ids}
+
+    with patch("scribe.services.system_usage.usage_for_systems",
+               AsyncMock(side_effect=_zeros)):
+        yield
+
+
 @pytest.fixture(autouse=True)
 def _no_task_log_arm():
     """Stub the task-log read arm that get_task / list_tasks / get_milestone