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/views/brews.py

@@ -145,7 +145,8 @@ def _start_advice(run_id: str, reading_id: str, corrected: bool = False) -> None
                 # reading will land in its own turn.
                 log.info(
                     "the advice for reading %s was not stored: the reading "
-                    "no longer holds the gravity it was written for",
+                    "has been edited since the advice was generated, so the "
+                    "advice no longer matches it",
                     read_id,
                 )
         except Exception:  # noqa: BLE001 - never let the thread die noisily