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_surfaces.py

@@ -5,9 +5,9 @@ WHY THIS EXISTS
 Six push arms each carried their own loose copy of the same shape: a settings
 key, a default, and a limit that was usually a module constant nobody could
 change. The read-and-clamp was written out separately in `plugin_context` (twice
-over, for auto-inject and the write path), in three rule arms, and again as
-`reply_preferences._threshold`. That was survivable while the numbers were
-shipped constants an operator occasionally edited.
+over, for auto-inject and the write path), in three rule arms, and again in
+the completion-report arm (since retired, milestone 500). That was survivable
+while the numbers were shipped constants an operator occasionally edited.
 
 It stops being survivable once the numbers are meant to MOVE. The operator's
 decision for this step: