fix(retrieval): the completion-report arm gets its own bar, not the prompt arm's (#3860)

`report_preference` shipped reading PROMPTRULE_THRESHOLD_KEY, so the two arms
were one dial: tuning the bar for an operator's prose silently retuned the
lookup that runs when a task closes.

That coupling is worse on this arm than it would be anywhere else. Every other
retrieval arm scores a query that varies per call, so a mis-set bar shows up as
a changed clear-rate. COMPLETION_QUERY is a fixed string, so this arm's best
score for a given corpus is a CONSTANT — and a constant sitting under the bar is
a dead arm rather than a quiet one. No volume of traffic reveals it.

Found by the first live read for milestone 394 step 9: 69 calls, 69 declines,
every one naming the same record at the same score (0.7194 against a 0.72 bar).
Reading `best_available_id` (#3807) showed the record was about interpreting a
REQUEST, not about report shape — so the declines were correct and the arm is
healthy. The percentile alone would have said "lower the bar", which would have
delivered a false positive on every completion report ever written.

The bar does not move; the key does. Both defaults stay 0.72, so this changes no
behaviour on any install — it makes "leave this one where it is" expressible,
which it was not before. Settings grows the control (rules 25, 27), and the
default-agreement check grows a row.

Deliberately NOT included: a change to PROMPTRULE_DEFAULT_THRESHOLD. The
evidence for moving it is this install's near-miss table, and rule 115 keeps a
shipped default from being justified by one instance's corpus. That bar is a
per-user setting and belongs in the operator's Settings, not in the product.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

frontend/src/views/SettingsView.vue

@@ -96,6 +96,7 @@ 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");
 // Near-duplicate report floors, one per record kind (services/dedup.py).
 // Snippets are single-chunk, so their floor sits below the 0.90 write-time
 // gate and catches what it lets through. Notes/tasks are scored at chunk
@@ -171,6 +172,7 @@ async function saveKbInject() {
   // Bash call, so a fallback of 0 would put a rule in front of every command.
   const trT = Math.min(1, Math.max(0, Number(kbToolRuleThreshold.value) || 0.68));
   const prT = Math.min(1, Math.max(0, Number(kbPromptRuleThreshold.value) || 0.72));
+  const rpT = Math.min(1, Math.max(0, Number(kbReportPrefThreshold.value) || 0.72));
   kbInjectThreshold.value = String(t);
   kbInjectTopK.value = String(k);
   kbDupThresholdSnippet.value = String(dupSnip);
@@ -181,6 +183,7 @@ async function saveKbInject() {
   kbRuleHintThreshold.value = String(rhT);
   kbToolRuleThreshold.value = String(trT);
   kbPromptRuleThreshold.value = String(prT);
+  kbReportPrefThreshold.value = String(rpT);
   savingKbInject.value = true;
   kbInjectSaved.value = false;
   try {
@@ -202,6 +205,12 @@ 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),
       kb_duplicate_threshold_snippet: String(dupSnip),
       kb_duplicate_threshold_note: String(dupNote),
       kb_duplicate_threshold_task: String(dupTask),
@@ -657,6 +666,9 @@ 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_writepath_threshold !== undefined) {
     kbWritePathThreshold.value = allSettings.kb_writepath_threshold;
   }
@@ -1568,6 +1580,28 @@ async function deleteUser(userId: number) {
             not a command or a file — which is why it carries its own number.
           </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. If you have written a preference for report shape
+            and it is not arriving, lower this; there is no run of calls that
+            will reveal the problem on its own.
+          </p>
+        </div>
         <!-- A design system belongs to a PROJECT, and the picker for it lives on
              the project. There was a setting here that designated the system
              this install's own interface was built from; it only ever described