fix(advice): the stripper must DROP the working, and the token is the guard

Two findings from the fourth review.

The stripper was a no-op in the case it was written for
---------------------------------------------------
The "all working" branch returned the text UNCHANGED, which is
byte-identical to the old fallthrough.  And in the case where a preamble
marker WAS present and no paragraph read like advice, it returned the
whole reply - working included - so the branch made the "store the
working instead of the advice" failure MORE likely, not less.

Measured on a realistic 5-paragraph reasoning reply: the old code stored
80 chars (one paragraph), the new code stored 612 chars (five
paragraphs) of the model's working.

It now DROPS the working, so the reply is empty and the brewer sees a
clear "could not be generated" rather than the model thinking about
their beer.

"keep" alone is not an advice marker
------------------------------------
"I need to keep in mind that the hydrometer was read at a different
temperature" is the model thinking, not advice.  The bare words "keep",
"raise", "lower", "take the" were matching ordinary reasoning phrasing,
so the "all working" detection almost never fired on real reasoning
text.  They are now matched as phrases - "you should keep", "raise it
to", "take another reading" - and "i need to keep in mind" is
explicitly working.

The token is the primary guard
------------------------------
The gravity can be unparseable for some readings, so `expected_sg` can
be None - and then the write was becoming unconditional, reintroducing
the stale-overwrite bug on those readings.

The editing token is the primary guard: it changes on EVERY edit, so it
catches a stale generation whatever else changed - the gravity, the
comment, or nothing.  The gravity is now a secondary check, used only
when there is no token.

177 tests (up from 174).

src/flask/mongo/repository.py

@@ -263,10 +263,14 @@ class BeerRepository:
             return 0
 
         query: dict = {"_id": {"$in": doc_id}}
-        if expected_sg is not None:
-            query["sg"] = str(expected_sg)
         if expected_token is not None:
+            # The editing token is the PRIMARY guard: it changes on every
+            # edit, so it catches a stale generation whatever else
+            # changed - the gravity, the comment, or nothing.
             query["advice_token"] = str(expected_token)
+        elif expected_sg is not None:
+            # No token - a direct call with a gravity to check against.
+            query["sg"] = str(expected_sg)
 
         # `$in` matches either form.  A plain list would be an exact
         # ARRAY match in Mongo, which is not what is wanted here.