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/plugin_context.py

@@ -601,25 +601,6 @@ PROMPTRULE_DEFAULT_THRESHOLD = SURFACES["prompt_rule"].floor_default
 # at all, where the act arms never face it because a command is one thing.
 PROMPTRULE_LIMIT = SURFACES["prompt_rule"].budget_default
 
-# 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 = SURFACES["report_preference"].floor_key
-REPORTPREF_DEFAULT_THRESHOLD = SURFACES["report_preference"].floor_default
-
 
 def _slugify(text: str) -> str:
     """kebab-case slug for a skill directory name (a-z0-9 + single hyphens)."""
@@ -726,7 +707,8 @@ async def get_autoinject_config(user_id: int) -> dict:
 
     THE TWO NUMBERS COME FROM THE REGISTRY NOW (#4102). They used to be read and
     clamped here, and identically again in `get_writepath_config`, and again in
-    three rule arms, and once more in `reply_preferences`. That was
+    three rule arms, and once more in the completion-report arm (since
+    retired, milestone 500). That was
     tolerable while the values were shipped constants. It stops being tolerable
     once a tool is expected to MOVE them, because a tuning surface cannot be
     consistent across arms that each spell their configuration differently.