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).

tests/test_brew_advice.py

@@ -649,15 +649,23 @@ def test_a_reply_that_is_all_working_stores_no_working():
     about their beer instead of any advice at all.
     """
     working_only = (
-        "Let me think about what this reading means.\n\n"
+        "Let me think about what this reading means. The original gravity is "
+        "1.055 and the current reading is 1.030, which is a drop of 25 points "
+        "over nine days at 19 degrees, and I need to work out what that means "
+        "for the expected attenuation.\n\n"
         "Perhaps the drop is smaller than expected, and it could be that the "
-        "yeast is stressed.\n\n"
+        "yeast is stressed by the temperature rather than actually done, "
+        "because most ale strains slow down considerably below 18 degrees.\n\n"
         "Wait, I need to keep in mind that the hydrometer was read at a "
-        "different temperature.\n\n"
+        "different temperature than the sample, which would make the reading "
+        "read lower than it really is, and I have no way of knowing from the "
+        "data alone what the correction factor was.\n\n"
         "I need to keep in mind that the original gravity here was measured "
-        "before the hops went in.\n\n"
-        "So the question is what the expected attenuation should be, and I "
-        "am not sure without more data."
+        "before the hops went in, and the reading on brew day was 1.055, so "
+        "the drop is twenty-five points against a potential of fifty-five.\n\n"
+        "So the question is what the expected attenuation should be for this "
+        "style at this temperature, and I am not sure without more data or a "
+        "temperature-corrected reading to compare against."
     )
 
     stripped = brew_advice._strip_reasoning(working_only)
@@ -667,6 +675,39 @@ def test_a_reply_that_is_all_working_stores_no_working():
     )
 
 
+def test_a_short_reply_is_advice_even_without_a_marker_word():
+    """Real advice is often phrased without any marker phrase.
+
+    "Actually the drop is fine. Wait a couple of days and check again."
+    is advice - and judging it paragraph by paragraph emptied it, so the
+    brewer saw nothing at all.
+    """
+    reply = ("Actually the drop is fine.\n\n"
+             "Wait a couple of days and check again.")
+
+    assert brew_advice._strip_reasoning(reply) == reply
+
+
+def test_a_long_reply_with_no_instruction_is_dropped():
+    """Long, and nothing in it reads like an instruction - all working."""
+    working = ("Let me think about this.\n\n" +
+               "Perhaps the yeast is stressed, and it could be that the "
+               "temperature is too low. " * 20)
+
+    assert brew_advice._strip_reasoning(working) == ""
+
+
+def test_a_long_reply_with_an_instruction_keeps_it():
+    """The working is trimmed from it, but the advice stays."""
+    mixed = ("Let me think about this.\n\n" +
+             "Perhaps the yeast is stressed. " * 15 +
+             "\n\nWarm the fermenter to 20C and give the yeast a gentle "
+             "swirl to rouse it.")
+
+    out = brew_advice._strip_reasoning(mixed)
+    assert "Warm the fermenter" in out
+
+
 def test_a_paragraph_about_keeping_something_in_mind_is_not_advice():
     """"Keep" alone is not an advice marker.
 
@@ -745,6 +786,46 @@ def test_the_advice_is_written_for_a_reading_that_predates_the_token(repo):
     assert after["comment"] == "a comment"
 
 
+def test_a_pre_token_reading_is_still_guarded_against_a_stale_write(repo):
+    """A reading that has never been edited has no token at all.
+
+    The guard matches on the ABSENCE of a token, so an edit still tells
+    the generations apart.  Falling back to the gravity here would
+    re-open the hole, because a comment-only correction leaves the
+    gravity unchanged.
+    """
+    repo.readings.insert_one({
+        "_id": "preToken0000000000000010",
+        "beer_run_id": "1",
+        "beer_type": "Ale",
+        "sg": "1050",
+        "date": "2026-11-01",
+        "final_reading": "False",
+        "comment": "first comment",
+    })
+
+    # The advice thread starts on this generation - no token.
+    assert repo.set_reading_advice("preToken0000000000000010", "FRESH ADVICE",
+                                    True, expected_sg=None,
+                                    expected_token="") == 1
+
+    # The brewer corrects the comment.  The edit writes a token.
+    repo.upsert_reading(beer_run_id="1", beer_type="Ale", sg=1050,
+                         reading_date="2026-11-01", final_reading=False,
+                         comment="corrected comment")
+
+    # The late thread for the OLD generation lands.  It carries no token,
+    # and the reading now HAS one - so it must be refused.
+    stale = repo.set_reading_advice("preToken0000000000000010", "STALE ADVICE",
+                                     True, expected_sg=None,
+                                     expected_token="")
+    assert stale == 0, "the stale advice overwrote the correction"
+
+    after = repo.readings.find_one({"beer_run_id": "1"})
+    assert after.get("advice") is None
+    assert after["comment"] == "corrected comment"
+
+
 def test_a_reply_cut_off_at_the_token_ceiling_is_marked(repo):
     """A reply that stopped at the token limit is incomplete.