feat(supersession): the relation, and the dead column that stood where it should

Step 1 of #278. Structure only — nothing reads or writes the new table yet.

Old records outrank newer ones on the same subject because a similarity score
cannot tell time. A note that accurately described how something worked in June
is still accurate ABOUT June; it is just no longer the answer. Nothing recorded
that, so nothing could act on it.

`note_supersessions(superseder_id, superseded_id)`. The claim points FORWARD —
the newer record names what it overtakes — because the older one cannot know it
has been overtaken; asking it to record its own obsolescence is asking it to
predict the future.

A table rather than a column because the relation is genuinely many-to-many and
partial, and both directions are hot: superseded_id answers "has this been
overtaken?" at ranking time, superseder_id answers "what does this replace?" in
a record view. An array column serves one and not the other.

CASCADE is safe because trashing is not a delete — trash_svc stamps deleted_at,
so a trashed note keeps its claims and restore brings them back. It fires only
on purge_trash, where a claim about the row would be unactionable anyway. A
CHECK rejects self-supersession, which under flat demotion would let a record
demote itself.

## consolidated_at, and what it actually was

Dropped. Written by nothing while serialised into every note and task payload
as null — and worse, it implied a capability.

The survey (#2483) read it as note consolidation modelled and abandoned. That
was wrong, and the frontend is what says so: `TaskViewerView` rendered
"✦ Auto-summarized from work logs" gated on this column. It is a survivor of
the pre-pivot auto-summary subsystem (migration 0030), whose own column #599
removed. Not an unbuilt feature — an outlived one.

So four more remnants went with it: the banner, its CSS, a `consolidatedAt` ref
in TaskEditorView assigned and never read, and `.auto-summary-banner-editor`
styling with zero template usage. That last one is presence-without-reference
in the same family as the column itself.

Dropped rather than repurposed for supersession, and the distinction is the
point: consolidation folds records into one survivor and destroys the
originals. Supersession is the opposite — both survive, the older ranks behind.
Smuggling one in under a column named for the other would bury that in schema.

## The hard delete_note

Removed, with a comment where it stood. Zero callers, and the danger was never
that it ran — it is that it was findable by name. Someone wanting to delete a
note greps `delete_note`, finds a function in the notes service with exactly
the right signature, and permanently destroys a record every path downstream
expects to be recoverable. The MCP tool of the same name already went through
trash_svc; only the service function was the trap.

Refs #278, #2483

frontend/src/views/TaskEditorView.vue

@@ -41,7 +41,6 @@ const toast = useToastStore();
 const title = ref("");
 const body = ref("");
 const description = ref("");
-const consolidatedAt = ref<string | null>(null);
 const tags = ref<string[]>([]);
 const status = ref<TaskStatus>("todo");
 const priority = ref<TaskPriority>("none");
@@ -303,7 +302,6 @@ onMounted(async () => {
       title.value = store.currentTask.title;
       body.value = store.currentTask.body;
       description.value = store.currentTask.description ?? "";
-      consolidatedAt.value = store.currentTask.consolidated_at ?? null;
       tags.value = [...(store.currentTask.tags || [])];
       status.value = store.currentTask.status as TaskStatus;
       priority.value = store.currentTask.priority as TaskPriority;
@@ -1063,22 +1061,4 @@ useEditorGuards(dirty, save);
   border-color: var(--color-primary);
 }
 
-/* ── Auto-summary banner + re-consolidate button ─────────────────────────── */
-.auto-summary-banner-editor {
-  display: flex;
-  align-items: center;
-  gap: 0.6rem;
-  padding: 0.45rem 0.7rem;
-  margin-bottom: 0.5rem;
-  font-size: 0.82rem;
-  font-style: italic;
-  color: var(--color-text-muted);
-  background: rgba(99, 102, 241, 0.06);
-  border-left: 2px solid var(--color-primary);
-  border-radius: var(--radius-sm);
-}
-.auto-summary-banner-editor .auto-summary-icon {
-  color: var(--color-primary);
-  font-style: normal;
-}
 </style>

frontend/src/views/TaskViewerView.vue

@@ -365,13 +365,6 @@ const subTaskProgress = computed(() => {
           <p class="goal-text">{{ store.currentTask.description }}</p>
         </div>
 
-        <div
-          v-if="store.currentTask.consolidated_at"
-          class="auto-summary-banner"
-        >
-          <span class="auto-summary-icon" aria-hidden="true">✦</span>
-          Auto-summarized from work logs.
-        </div>
 
         <div
           class="body prose"
@@ -771,17 +764,4 @@ const subTaskProgress = computed(() => {
   color: var(--color-text);
   white-space: pre-wrap;
 }
-.auto-summary-banner {
-  display: flex;
-  align-items: center;
-  gap: 0.5rem;
-  font-size: 0.78rem;
-  font-style: italic;
-  color: var(--color-text-muted);
-  margin: 0 0 0.75rem;
-}
-.auto-summary-icon {
-  color: var(--color-primary);
-  font-size: 0.85rem;
-}
 </style>