docs(notes): the surfaces say WHEN a note earns a check, and a guard keeps them saying it (#3168, milestone 317 step 5)

The step that decides whether steps 1-4 were worth building. `verify_with` is
a free-text field on the highest-volume record kind in the product; described
only as "how to verify this note" it gets filled in on every note within a
week, and at that point the sweep returns the whole corpus and means nothing.
The signal is not "has a check" — it is "has a check AND almost nothing else
does".

Rule 119 puts this in the app's own instruction surfaces, never in a Scribe
rule. So:

- the using-scribe skill gains the reflex, next to "state updates in place;
  chronicles don't" — its other half, since supersession only fires once
  somebody has read a note and disagreed, which is the case where it was
  already believed;
- update_note's docstring now states the DEFAULT rather than only deferring
  to create_note for the test. Found by the new guard on its first run;
- plugin 0.1.48.

THE _INSTRUCTIONS BUDGET, decided rather than skipped. The payload is ~1980 of
the client's ~2048-char cap, so everything in it competes for the last ~68
characters. The operator declined a line for this milestone: the map's own
closing sentence says each tool's description carries the full contract, and
the sweep is a curation act rather than a session-start reflex like
enter_project or list_always_on_rules. That reasoning is now a comment beside
the constant, with the accepted cost named — an agent that never opens
create_note's docstring never learns the field exists — so the question is not
re-litigated blind.

The guard pins STRUCTURE, never wording, for the disambiguator's reason (a
test that punishes rewriting is a test that gets deleted): each write surface
must still draw the norm-vs-constraint distinction, say the empty case is
normal, and name where NOT to reach for it — plus that the skill carries the
one-question form, because the docstrings only reach a caller who already
opened the tool.

`_doc` moves to tests/helpers as `tool_doc`; it had been written twice.

src/scribe/mcp/tools/notes.py

@@ -224,10 +224,12 @@ async def update_note(
         supersedes: Replace the ids of earlier notes this one replaces
             (set-semantics). None = leave unchanged; [] = clear all. See
             create_note for when to reach for it.
-        verify_with: How to check this note is still true. See create_note for
-            the norm-vs-constraint test that decides whether it should carry
-            one at all; the short form is "could this become false without
-            anyone editing it?".
+        verify_with: How to check this note is still true. Almost every note
+            should leave this empty — that is the normal, finished state, not
+            a gap: an empty check is the marker for "this is a decision, there
+            is nothing to go and check". See create_note for the full
+            norm-vs-constraint test; the short form is "could this become
+            false without anyone editing it?".
         expires_when: The STATE that ends it, not a date.
         clear: Names of fields to UNSET — "verify_with", "expires_when".
             Needed because "" means "leave this alone" here, so there is no