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

src/scribe/services/plugin_context.py

@@ -471,6 +471,25 @@ PROMPTRULE_DEFAULT_THRESHOLD = 0.72
 # at all, where the act arms never face it because a command is one thing.
 PROMPTRULE_LIMIT = 3
 
+# THE COMPLETION-REPORT ARM'S OWN BAR (services/reply_preferences.py).
+#
+# It borrowed PROMPTRULE_THRESHOLD_KEY when it shipped, which made the two
+# arms one dial: an operator lowering the bar for their own prose moved this
+# one with it, silently. That contradicts the rule every other bar here
+# follows — one number cannot serve arms whose queries are different shapes —
+# and this arm's query is the most different of all. The others score an
+# operator's prose or a session's code, both of which vary per call; this one
+# scores a FIXED string (`COMPLETION_QUERY`) against rule triggers, so its
+# score for a given corpus is a constant. A constant that lands under the bar
+# is not a quiet arm, it is a dead one, and nothing about the prose arm's
+# traffic would ever reveal it.
+#
+# Kept at the prose arm's starting value rather than tuned: the split is what
+# makes the two independently movable, and a default is a product decision
+# that this install's corpus cannot settle (rule 115).
+REPORTPREF_THRESHOLD_KEY = "kb_reportpref_threshold"
+REPORTPREF_DEFAULT_THRESHOLD = 0.72
+
 
 def _slugify(text: str) -> str:
     """kebab-case slug for a skill directory name (a-z0-9 + single hyphens)."""