feat(notes): a note's check is writable through both doors, and empty means empty (#3164, milestone 317 step 2)

The rules path's three lessons (#3096), inherited:

EMPTY MEANS NULL. The sweep's whole signal is `verify_with IS NULL` = "this is
a decision, there is nothing to go and check". A "" that is not NULL makes a
norm look like a constraint nobody has verified — and never-checked sorts
FIRST, so it would sit at the top of the sweep forever.

CLEARING IS EXPLICIT. At the MCP door "" means "leave this alone", so an agent
updating a body does not wipe a check it was never asked about — which leaves
no value meaning "remove it". `clear` names the field, and naming it cannot
happen by accident. The REST door, where a cleared form input arrives as "",
reaches the same place through normalisation: two idioms, one outcome.

THE STAMP CERTIFIES A CHECK, NOT A RECORD. Rewrite or clear `verify_with` and
`verified_at` is dropped, so the note re-enters the sweep. A note wrongly
listed as due costs one look; a note wrongly vouched for costs exactly what
the sweep exists to catch. `verified_at` is also no longer settable through an
ordinary edit — a stamp says somebody performed THIS check, and minting one
from a write that ran no check is the one thing that would make the sweep lie.

And one this path adds: not every record may carry a check. A task's decay is
its status — a done issue records what happened rather than asserting
something that can go false — and a snippet already has verify_snippet, which
compares its recorded location and code against the repo and expires its own
verdict. Both are refused with a message naming the alternative, never dropped
silently (minted_kind's reasoning, #3129), and the gate lives at the service
so the two doors cannot come to disagree.

Written as an INVARIANT over the resulting record, not a filter on which
fields were passed. That is what catches the sideways route — a checked note
being turned into a task, a write that names no check at all and would sail
past any per-field gate.

The MCP docstrings carry the norm-vs-constraint test, because at that door the
docstring IS the contract and a field described only as "how to verify this"
gets filled in on every note. Step 5 does this properly across the instruction
surfaces; this is the minimum that stops the field being misused on arrival.

tests/helpers gains `drive_update_note` — the patch stack for driving
update_note, written twice before this and now once. The note-shaped fakes
gain the trio explicitly, for fake_note's own stated reason: an unset
attribute is a truthy MagicMock, and a truthy verify_with reads as a check
that is there.

src/scribe/mcp/tools/notes.py

@@ -103,6 +103,8 @@ async def create_note(
     project_id: int = 0,
     system_ids: list[int] | None = None,
     supersedes: list[int] | None = None,
+    verify_with: str = "",
+    expires_when: str = "",
     force: bool = False,
 ) -> dict:
     """Create a new note in Scribe.
@@ -134,6 +136,24 @@ async def create_note(
             arrives labelled when it does surface. This records a CLAIM, not a
             verdict: it never says the older note was wrong, only that it is no
             longer the current answer.
+        verify_with: HOW TO CHECK this note is still true. Leave empty for
+            almost every note — that is the normal case, not an unfinished
+            one.
+            A note is a NORM or a CONSTRAINT. A norm is a decision ("we derive
+            versions from commit time"): it has no truth value and changes only
+            when its author changes it, which they know they did. A CONSTRAINT
+            asserts a fact about someone else's software ("AMO refuses to
+            re-sign a version", "this forge numbers CI runs per repository"),
+            and it goes false with nobody watching. Only constraints get a
+            check.
+            The test, in one question: COULD THIS NOTE BECOME FALSE WITHOUT
+            ANYONE EDITING IT? If no, leave this empty.
+            A command, a path, a URL, a query. Prose is allowed; something
+            runnable is better.
+        expires_when: The STATE that ends it — deliberately not a date.
+            "When Forgejo issues run numbers per workflow rather than per
+            repository", not "in six months". Constraints expire when the
+            ground moves, not on a schedule.
         force: Bypass the near-duplicate gate. By default, if a title- or
             meaning-similar note already exists in the same project, creation is
             BLOCKED and the existing note's id is returned so you update it
@@ -161,6 +181,8 @@ async def create_note(
         body=body,
         tags=tags,
         project_id=project_id or None,
+        verify_with=verify_with,
+        expires_when=expires_when,
     )
     if system_ids:
         await systems_svc.set_record_systems(uid, note.id, system_ids)
@@ -185,6 +207,9 @@ async def update_note(
     project_id: int = 0,
     system_ids: list[int] | None = None,
     supersedes: list[int] | None = None,
+    verify_with: str = "",
+    expires_when: str = "",
+    clear: list[str] | None = None,
 ) -> dict:
     """Update an existing Scribe note. Only explicitly provided fields are changed.
 
@@ -199,6 +224,25 @@ 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?".
+        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
+            value that removes a field: an agent updating a body must not
+            silently wipe a check it was not asked about. A note that stops
+            being a constraint is cleared by naming the field, which cannot
+            happen by accident.
+
+    Rewriting `verify_with` drops the note's verification stamp: a stamp
+    certifies a particular check, and carrying it across a rewrite would vouch
+    for something nobody has looked at.
+
+    A task and a snippet are both REFUSED a check, with a message saying where
+    to go instead — a task's decay is its status, and a snippet has
+    verify_snippet.
     """
     uid = current_user_id()
     fields: dict = {}
@@ -210,7 +254,13 @@ async def update_note(
         fields["tags"] = tags
     if project_id:
         fields["project_id"] = project_id
-    note = await notes_svc.update_note(uid, note_id, **fields)
+    if verify_with:
+        fields["verify_with"] = verify_with
+    if expires_when:
+        fields["expires_when"] = expires_when
+    note = await notes_svc.update_note(
+        uid, note_id, clear=clear or (), **fields
+    )
     if note is None:
         raise ValueError(f"note {note_id} not found")
     if system_ids is not None: