feat(500): retire the fixed-question preference arm - completion-report preferences ride the moments

report_preference searched one constant string when a task closed and handed
matches back as update_task's reply_preferences. Milestone 500 step 3 delivers
the reply shapes and the preferences mounted beside them on the moments, so
the arm goes, whole (rule 22):

- services/reply_preferences.py, the REPORT_PREFERENCE RuleArm, its re-scorer
  and corpus entries, and the REPORTPREF constants
- update_task's reply_preferences block and its cue; the docstring now points
  at moment_rules
- the fixed_query field and the fixed_query_never_clears warning: this arm was
  the only one that set it, so the concept and the cannot_decline exemption
  go with it
- the two Settings fields for its floor and budget
- migration 0121 deletes its rows (logs, judgments, rule-usage events, tuning
  history, settings keys), operator-approved 2026-10-09. Without it the
  readout would call the source unregistered and its surfacings would count
  as ambient. The unindexed rule_usage delete is bounded by created_at.

#5496 (step 4 of milestone 500).

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

src/scribe/services/retrieval_pipeline.py

@@ -421,10 +421,6 @@ class Declared:
     quiet_because: str = ""
     """Set when silence over an active window is correct, saying why (#2475)."""
 
-    fixed_query: bool = False
-    """The arm always searches the same string, so its decline rate is 0% or
-    100% and `cannot_decline` says nothing about it."""
-
 
 @dataclass(frozen=True)
 class RankedSource:
@@ -523,37 +519,6 @@ WRITE_PATH_RULE = RuleArm(
     ),
     declared=Declared("rules that may govern the file being written"),
 )
-# The completion report's preferences (milestone 409 step 4): a FIXED query,
-# preferences only, read by update_task as records rather than as lines. Its
-# query is `reply_preferences.COMPLETION_QUERY`.
-REPORT_PREFERENCE = RuleArm(
-    "report_preference", band=False, compact_tail=False, checkpoint=False,
-    preference_slot=False, kind="preference",
-    tuning=Surface(
-        name="report_preference",
-        floor_key="kb_reportpref_threshold",
-        floor_default=0.72,
-        budget_key="kb_reportpref_top_k",
-        budget_default=3,
-        # THE ONE FIXED QUERY, and the reason this arm behaves unlike the rest.
-        # The others score something that varies per call; this one scores a
-        # constant string, so its top score for a given corpus is also a
-        # constant. A floor a hair above that constant is not a quiet arm, it
-        # is a dead one, and no amount of traffic will ever reveal it — which
-        # is precisely how this arm spent 69 calls declining the same record.
-        asks="a fixed question about how to lay out a completion report",
-        over="preferences",
-        fires="when a task finishes",
-    ),
-    # `fixed_query`: COMPLETION_QUERY is a module constant, so this arm's top
-    # score is the same number on every call — measured at 0.791 across 45
-    # consecutive calls, with p10, p50, p90, min and max all identical. Five
-    # equal percentiles is the tell.
-    declared=Declared(
-        "the fixed question asked when a task finishes: how should this report read",
-        fixed_query=True,
-    ),
-)
 # The backstop for every arm that ran earlier in the turn and missed: the
 # finished reply against every rule's trigger. Its floor IS its stop bar
 # (the reply_rule surface), so it is passed as both.
@@ -581,7 +546,7 @@ REPLY_RULE = RuleArm(
     ),
 )
 RULE_ARMS: tuple[RuleArm, ...] = (
-    WRITE_PATH_RULE, PRE_TOOL_RULE, PROMPT_RULE, REPORT_PREFERENCE, REPLY_RULE,
+    WRITE_PATH_RULE, PRE_TOOL_RULE, PROMPT_RULE, REPLY_RULE,
 )
 
 PREFERENCE_SLOT_SOURCE = "preference_slot"