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>

frontend/src/views/SettingsView.vue

@@ -168,7 +168,6 @@ const kbToolRuleThreshold = ref("0.68");
 // And the prompt boundary is a third query shape again — the operator's own
 // prose rather than anything a tool produced (#3852).
 const kbPromptRuleThreshold = ref("0.72");
-const kbReportPrefThreshold = ref("0.72");
 // The reply arm (milestone 458): its bar is the bar at which a finished reply
 // is HELD for one read, so it sits with the checkpoint, not with the hints.
 const kbReplyRuleThreshold = ref("0.8");
@@ -181,7 +180,6 @@ const kbWritePathTopK = ref("3");
 const kbRuleHintTopK = ref("5");
 const kbToolRuleTopK = ref("5");
 const kbPromptRuleTopK = ref("3");
-const kbReportPrefTopK = ref("3");
 const kbReplyRuleTopK = ref("1");
 // What has been changed about retrieval, newest first — the review surface for
 // changes the model made on the operator's behalf (#4102).
@@ -387,7 +385,6 @@ async function saveKbInject() {
   // this is the only number that can STOP a call, so a fallback of 0 would
   // hold the first command of every session behind whatever ranked first.
   const cpT = asBar(kbCheckpointThreshold.value, 0.8);
-  const rpT = asBar(kbReportPrefThreshold.value, 0.72);
   // A stop bar like the checkpoint's: a fallback of 0 would hold every reply.
   const ryT = asBar(kbReplyRuleThreshold.value, 0.8);
   // The budgets, clamped the way the server clamps them: a whole number in
@@ -400,13 +397,11 @@ async function saveKbInject() {
   const rhK = asK(kbRuleHintTopK.value, 5);
   const trK = asK(kbToolRuleTopK.value, 5);
   const prK = asK(kbPromptRuleTopK.value, 3);
-  const rpK = asK(kbReportPrefTopK.value, 3);
   const ryK = asK(kbReplyRuleTopK.value, 1);
   kbWritePathTopK.value = String(wpK);
   kbRuleHintTopK.value = String(rhK);
   kbToolRuleTopK.value = String(trK);
   kbPromptRuleTopK.value = String(prK);
-  kbReportPrefTopK.value = String(rpK);
   kbReplyRuleTopK.value = String(ryK);
   kbInjectThreshold.value = String(t);
   kbInjectTopK.value = String(k);
@@ -424,7 +419,6 @@ async function saveKbInject() {
   kbCheckpointThreshold.value = String(cpT);
   kbToolRuleThreshold.value = String(trT);
   kbPromptRuleThreshold.value = String(prT);
-  kbReportPrefThreshold.value = String(rpT);
   kbReplyRuleThreshold.value = String(ryT);
   savingKbInject.value = true;
   kbInjectSaved.value = false;
@@ -453,12 +447,6 @@ async function saveKbInject() {
       // queries are different shapes. Moving one must not move the others.
       kb_toolrule_threshold: String(trT),
       kb_promptrule_threshold: String(prT),
-      // A SIXTH, and the one that most needed its own key: this arm's query
-      // is a fixed string, so its score is a constant for a given corpus.
-      // While it borrowed the prompt bar, tuning prose silently retuned it —
-      // and a constant that lands under the bar is a dead arm, not a quiet
-      // one (#3860).
-      kb_reportpref_threshold: String(rpT),
       // The reply backstop's own bar: it HOLDS a reply, so like the
       // checkpoint it is never derived from a hint bar.
       kb_replyrule_threshold: String(ryT),
@@ -470,7 +458,6 @@ async function saveKbInject() {
       kb_rulehint_top_k: String(rhK),
       kb_toolrule_top_k: String(trK),
       kb_promptrule_top_k: String(prK),
-      kb_reportpref_top_k: String(rpK),
       kb_replyrule_top_k: String(ryK),
       kb_duplicate_threshold_snippet: String(dupSnip),
       kb_duplicate_threshold_note: String(dupNote),
@@ -941,9 +928,6 @@ onMounted(async () => {
   if (allSettings.kb_promptrule_threshold !== undefined) {
     kbPromptRuleThreshold.value = allSettings.kb_promptrule_threshold;
   }
-  if (allSettings.kb_reportpref_threshold !== undefined) {
-    kbReportPrefThreshold.value = allSettings.kb_reportpref_threshold;
-  }
   if (allSettings.kb_replyrule_threshold !== undefined) {
     kbReplyRuleThreshold.value = allSettings.kb_replyrule_threshold;
   }
@@ -967,9 +951,6 @@ onMounted(async () => {
   if (allSettings.kb_promptrule_top_k !== undefined) {
     kbPromptRuleTopK.value = allSettings.kb_promptrule_top_k;
   }
-  if (allSettings.kb_reportpref_top_k !== undefined) {
-    kbReportPrefTopK.value = allSettings.kb_reportpref_top_k;
-  }
   if (allSettings.kb_replyrule_top_k !== undefined) {
     kbReplyRuleTopK.value = allSettings.kb_replyrule_top_k;
   }
@@ -1974,42 +1955,6 @@ async function deleteUser(userId: number) {
           />
           <p class="field-hint">How many rules or preferences one message may be shown (1–10).</p>
         </div>
-        <div class="field">
-          <label for="kb-reportpref-threshold">Completion-report confidence threshold (0–1)</label>
-          <input
-            id="kb-reportpref-threshold"
-            v-model="kbReportPrefThreshold"
-            type="number"
-            min="0"
-            max="1"
-            step="0.01"
-            class="fs-input input"
-            style="max-width: 8rem"
-          />
-          <p class="field-hint">
-            The bar for a <em>preference about how a completion report should be
-            written</em>, looked up when a task closes. Unlike every other bar
-            here, the question this arm asks never changes — so its score is
-            fixed by your preferences alone, and it will either always find one
-            or never find one, and no run of calls will reveal a dead one on its
-            own. That is why this arm is worth looking up in the panel below when
-            a report preference never seems to arrive.
-          </p>
-        </div>
-        <div class="field">
-          <label for="kb-reportpref-topk">Max report preferences</label>
-          <input
-            id="kb-reportpref-topk"
-            v-model="kbReportPrefTopK"
-            type="number"
-            min="1"
-            max="10"
-            step="1"
-            class="fs-input input"
-            style="max-width: 8rem"
-          />
-          <p class="field-hint">How many preferences a finished task may be shown (1–10).</p>
-        </div>
         <div class="field">
           <label for="kb-replyrule-threshold">Reply check threshold (0–1)</label>
           <input