fix(advice): keep the advice when the reply is over-long

The review found a way to store the model's WORKING instead of its
advice.

The advice is at the END of the reply, so the length cap was cutting
from the front - which took the advice off and left only the working.
That is worse than no advice at all, because it looks like advice: the
brew sees a full paragraph of the model thinking about their beer, and
nothing about what to do.

Reproduced:

    input len: 2515 -> final len: 1199
    advice present? False
    TAIL: 'padding padding padding padding padding'

Two changes:

- The length cap keeps the END of the reply and cuts at a sentence
  boundary near the end, so the advice survives.
- The stripper now also catches a reply that is entirely working - no
  marker phrase, and no paragraph that reads like advice - instead of
  only catching one that says "let me think".

    input: 2515 -> stripped: 2515 -> final: 1199
    advice present? True

A short reply is untouched.

171 tests (up from 169).

tests/test_brew_advice.py

@@ -258,6 +258,33 @@ def test_a_reply_that_starts_with_the_advice_is_untouched():
     assert brew_advice._strip_reasoning(reply) == reply
 
 
+def test_an_over_long_reply_keeps_the_advice_at_the_end():
+    """The advice is at the END of the reply, so a plain truncation from
+    the front would cut it off and leave only the model's working.
+
+    That is worse than no advice at all, because it looks like advice.
+    """
+    working = (
+        "I need to work through what this reading means. The original gravity "
+        "is 1.055 and the current reading is 1.030. " + ("padding " * 260) +
+        "Leave the fermenter alone, take another reading in three days and "
+        "only then consider repitching."
+    )
+
+    final = brew_advice._trim_advice(working)
+
+    assert len(final) <= brew_advice.MAX_ADVICE_CHARS
+    assert "Leave the fermenter alone" in final, (
+        "the advice was cut off and only the model's working was left"
+    )
+
+
+def test_a_short_reply_is_not_trimmed():
+    """The trim must not touch a reply that already fits."""
+    reply = "The gravity looks fine for day nine. Give it three more days."
+    assert brew_advice._trim_advice(reply) == reply
+
+
 def test_the_advice_is_truncated():
     """A runaway reply must not fill the reading document."""
     long_text = "x" * 5000