fix(advice): the pre-token guard, and a stripper that cannot empty real advice

Two findings from the fifth review.

The pre-token fallback re-opened the hole
-----------------------------------------
A reading that has never been edited has no token.  The guard fell back
to the gravity check for those, and a comment-only correction leaves the
gravity unchanged - so a late thread's write matched and overwrote the
fresh advice, for exactly the 47 existing readings.

Reproduced live before the fix:

    pre-token reading token: None  ->  stale thread carries ""
    fresh write modified: 1
    stale write modified: 1   <<< stale advice overwrote the fresh one
    final advice: 'STALE ADVICE'

The guard now matches on the ABSENCE of a token, which still tells the
generations apart: an edit writes a token, so a stale thread's "" stops
matching the moment the reading is corrected.

    fresh write modified: 1 (must be 1)
    stale write modified: 0   (must be 0)

The stripper could empty real advice
------------------------------------
"Actually the drop is fine.\\n\\nWait a couple of days and check again."
is advice, and judging it paragraph by paragraph emptied it - so the
brewer saw nothing at all, with no failure note.

The test is no longer a marker list, which real advice often does not
contain.  A reply is all-working when it is LONG and has no instruction
in it: the model's working runs to thousands of characters, while the
advice is two to five sentences.

    short reply with no marker  -> kept
    long reply, no instruction  -> dropped
    long reply with advice      -> advice kept
    single-paragraph working    -> dropped

Also: the log message named the gravity, but the guard is the token.

182 tests (up from 178).

src/flask/brew_advice.py

@@ -276,7 +276,10 @@ def _strip_reasoning(text: str) -> str:
     # be a no-op, and the brewer would be shown the model thinking about
     # their beer instead of any advice at all.
     paragraphs = [p.strip() for p in text.split("\n\n") if p.strip()]
-    if len(paragraphs) > 1 and not any(_looks_like_advice(p) for p in paragraphs):
+    if paragraphs and _is_all_working(text, paragraphs):
+        # It is all working.  It is dropped, so the brewer sees a clear
+        # "could not be generated" rather than the model thinking about
+        # their beer.
         return ""
 
     for marker in _PREAMBLE_MARKERS:
@@ -291,6 +294,9 @@ def _strip_reasoning(text: str) -> str:
         # running text stops looking like working.
         paragraphs = [p.strip() for p in tail.split("\n\n") if p.strip()]
         if len(paragraphs) < 2:
+            # A single paragraph: the whole tail is one block of working
+            # with the advice inside it, so it is kept whole rather than
+            # thrown away.
             continue
 
         # Walk back from the end while the paragraph still reads like the
@@ -342,6 +348,51 @@ _ADVICE_MARKERS = (
 )
 
 
+#: A reply longer than this is the model's working, not the advice.
+#: The advice is meant to be two to five sentences.
+_WORKING_LENGTH = 600
+
+
+def _is_all_working(text: str, paragraphs: list[str]) -> bool:
+    """True when the reply is the model's working rather than advice.
+
+    Two signals, because either alone is unreliable:
+
+    - **Length.** The working runs to thousands of characters; the
+      advice is two to five sentences.
+    - **Shape.** The working interprets the numbers ("perhaps the yeast
+      is stressed, it could be that..."); the advice tells the brewer
+      what to do.
+
+    A reply that is SHORT is kept even if it has no marker word - real
+    advice is often phrased without one ("Actually the drop is fine.
+    Wait a couple of days and check again.").
+    """
+    if len(text) <= _WORKING_LENGTH:
+        # Too short to be the model's working.  It is advice, whatever
+        # it is worded like.
+        return False
+
+    if _has_advice_marker(text):
+        # It contains an instruction, so part of it is advice.
+        return False
+
+    # Long, and nothing in it reads like an instruction.
+    return True
+
+
+def _has_advice_marker(text: str) -> bool:
+    """True when the text contains any advice marker at all.
+
+    Used on the WHOLE reply rather than paragraph by paragraph, because
+    advice can be phrased without any of the markers - "Actually the
+    drop is fine.\n\nWait a couple of days and check again." is advice,
+    and judging it paragraph by paragraph emptied it.
+    """
+    lowered = text.lower()
+    return any(marker in lowered for marker in _ADVICE_MARKERS)
+
+
 def _looks_like_advice(paragraph: str) -> bool:
     """True when a paragraph reads like advice to the brewer.