feat(moments): the reply moment holds a finished reply for one read (milestone 458 step 4b, #4922)

The reply is the one act no tool call marks, and it is where "let me
know if it works" gets said. A new Stop hook (scribe_reply_check.sh)
sends the finished reply to POST /api/plugin/reply-rules, which checks
it twice:

- mounted: every unopened RULE on reply.report, plus reply.ask when the
  reply asks a question. Deterministic.
- semantic: the reply's head and tail against every rule's trigger, on a
  new ranked surface, reply_rule. It is the backstop for whatever the
  earlier arms missed. Its floor is its stop bar (default 0.80, budget
  1), with its own Settings dials. The new stop_only stage records
  surfacing for the rule that holds and nothing else, because nothing
  else reached anyone.

Following the operator's ruling from 456 step 8, a rule that holds blocks
once, in the server's words. The hook blocks only on a reason it was
given, so an unreachable instance never stops a session, and it never
holds the rewrite. The ledger is the act checkpoint's own, so a rule
holds a session once across both doors and the per-session cap counts
both.

The turn reader moved from the report check into scribe_defs.sh
(scribe_turn_facts / scribe_turn_fact), so the two Stop hooks read a
turn the same way. The output was checked identical on a real transcript.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/services/retrieval_surfaces.py

@@ -207,6 +207,21 @@ SURFACES: dict[str, Surface] = {
         over="preferences",
         fires="when a task finishes",
     ),
+    # THE REPLY BACKSTOP (milestone 458, folded in from 456 step 8). Its floor
+    # is a STOP bar, not a hint bar: at the end of a turn nothing can be shown
+    # beside the reply, so a hit either holds the reply for one read or says
+    # nothing. Hence a default at the checkpoint's level and a budget of one —
+    # the call row's results are then exactly the rule that would hold.
+    "reply_rule": Surface(
+        name="reply_rule",
+        floor_key="kb_replyrule_threshold",
+        floor_default=0.80,
+        budget_key="kb_replyrule_top_k",
+        budget_default=1,
+        asks="the reply that ends a turn, against rule triggers",
+        over="global rules plus the bound project's own",
+        fires="once per turn, when the reply is finished",
+    ),
 }
 
 # Reserved slots are deliberately absent. `preference_slot`, `reuse_slot` and