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>

src/scribe/services/retrieval_registry.py

@@ -34,8 +34,8 @@ every tunable surface also appears here, so the two cannot drift apart.
 ADDING AN ARM MEANS ADDING A ROW HERE. `tests/test_retrieval_registry.py`
 walks the call sites with the ast module and fails on a source it cannot find
 below — deliberately not a grep, because two of the sources in this file
-(`wide_net`, `report_preference`) reach their recorder as `source=SOURCE`
-through a module constant and a grep for `source="` misses both. That is the
+(`wide_net`, `preference_slot`) reach their recorder through a
+module constant and a grep for `source="` misses both. That is the
 narrowing #3191 warns about, caught here in the act.
 """
 from __future__ import annotations