fix(docs): two comments cited the wrong snippet for the duplication (#4102)

Both said the repeated read-and-clamp was "canon #2860 across 295 of 372
siblings". Wrong on both halves, and checked rather than assumed:
#2860 is "Scribe service function — the async_session unit", the
service-layer canon, and its instance count is that canon's dominance in
src/scribe/services — nothing to do with a threshold helper. The
divergence hook flagged the new module against it because the new module
is a service, which is the hook working correctly and me misreading it.

The duplication itself was real and is what the registry consolidated:
`plugin_context` read and clamped the pair twice over, three rule arms
did it again, and `reply_preferences._threshold` once more. The comments
now say that, and claim nothing about a ledger count.

`rule_usage.py:122` cites #2860 correctly as the service canon and is
untouched.

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

@@ -618,8 +618,7 @@ 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` — the duplication the
-    shape ledger counts as canon #2860 across 295 of 372 siblings. That was
+    three rule arms, and once more in `reply_preferences`. 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.

src/scribe/services/retrieval_surfaces.py

@@ -4,9 +4,10 @@ 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. `_threshold()` was duplicated so widely that the shape ledger counts it
-as canon across 295 of 372 judged siblings (snippet #2860). That was survivable
-while the numbers were shipped constants an operator occasionally edited.
+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.
 
 It stops being survivable once the numbers are meant to MOVE. The operator's
 decision for this step: