feat(tasks): task_kind gains 'spike' — the investigation, not the change (#3099, milestone 312 step 5)

A spike is a shape the other kinds cannot hold. `work` ships a change;
`issue` fixes something broken. A spike is time-boxed and its output is
KNOWLEDGE — it succeeds by producing an answer, and nothing ships at the end
of it. Filing one as `work` makes a finished investigation look like an
abandoned change, which is why the distinction earns a value rather than a
convention.

It is also the record a failed check asks for. This milestone gave rules a
verify_with; when one fails the rule is wrong, and the next move is often to
go and find out what replaced it. notes.arose_from_id already exists (0065),
so constraint -> spike provenance needed no schema at all — only a docstring
saying it is there.

Rule 36: the value and the widened CHECK land in the same migration, DROP
then ADD, exactly as 0065 did for 'issue'. The two whitelists live in one
tuple each so upgrade and downgrade cannot disagree about what the list was
on either side. The downgrade demotes existing spikes to 'work' first —
lossy, deliberately, because the alternative is a downgrade that fails on
real data, and one that says what it did beats one that cannot run.

'plan' stays whitelisted though retired: historical plan-tasks carry it, and
a row that cannot be rewritten cannot be edited, restored or migrated.

The integration test asserts both halves. A test that only proved 'spike' is
accepted would pass just as happily against a table whose CHECK had been
dropped and never re-added — which is the other way rule 36's failure
happens — so an unknown kind is asserted to still raise.

Not in scope, deliberately: any special lifecycle, time-box enforcement, or
gating relationship. It is a kind, not a workflow.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/scribe/models/note.py

@@ -61,10 +61,16 @@ class Note(Base, TimestampMixin, SoftDeleteMixin):
     # Note type — 'note' (default) or 'process' (a stored process). Task-ness is
     # tracked by `status`, not here. (person/place/list entity types removed 2026-07.)
     note_type: Mapped[str] = mapped_column(Text, default="note", server_default="note")
-    # Task sub-kind — 'work' (default), 'plan', or 'issue' (corrective work).
+    # Task sub-kind — what KIND of work this is, not how it is going:
+    #   work   (default) — ships a change
+    #   issue  — corrective; something was broken (0065)
+    #   spike  — time-boxed, and its output is KNOWLEDGE rather than a change;
+    #            it succeeds by producing an answer, and nothing ships (0091)
+    #   plan   — retired since 0066 (plans are milestones), kept in the CHECK
+    #            so historical plan-tasks stay writable
     # Only meaningful when the note is a task (status is not None); ordinary
     # notes keep the 'work' default and ignore it. Orthogonal to note_type
-    # (which is the note/entity axis).
+    # (which is the note/entity axis). CHECK notes_task_kind_check (rule 36).
     task_kind: Mapped[str] = mapped_column(Text, default="work", server_default="work")
     # Queryable structured fields for typed records — currently snippets, whose
     # name/language/signature/locations live here so they can be INDEXED. The