refactor(retrieval): the completion-report preferences run through the pipeline (milestone 456 step 3, #4905)

reply_preferences.completion_preferences was the last hand-written
rule search + record_retrieval + record_rule_surfaced triple outside the
pipeline. It is now REPORT_PREFERENCE, a RuleArm with kind="preference"
over COMPLETION_QUERY, read back as records rather than as lines.

- RuleArm gains `kind`. _ranked asks the ranker for that kind and drops
  anything else on the way out, before the row is logged.
- RuleResult gains `shown`, the (score, rule) pairs behind the lines, for
  callers that return records.
- RuleMoment.project_id may be None: logged as given, searched as
  `project_id or None`. report_preference rows keep their NULL project.
- Guards: reply_preferences now has 0 direct rule searches. The registry
  constant-source example moves from report_preference to preference_slot,
  the pipeline slot that records through a module constant.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_rule_usage_wiring.py

@@ -1702,7 +1702,7 @@ def test_every_hook_rule_search_says_which_project_it_is_for():
         # ranked-search stage, checked below — so a direct search appearing
         # here again is a copy of the arm coming back, and fails the count.
         "src/scribe/services/plugin_context.py": 0,
-        "src/scribe/services/reply_preferences.py": 1,
+        "src/scribe/services/reply_preferences.py": 0,
     }
     for path, expected in sources.items():
         calls = [