feat(dedup): the duplicate report reaches notes and tasks, with per-kind cures

Step 5 of #278, folding in #2534. The operator's no-gate decision for the web
UI (#2482 — "an llm attached to this surface is the corrections system") has a
precondition nobody had built: the corrector has to be able to SEE what needs
correcting. find_duplicate_snippets had no equivalent for notes or tasks, so a
duplicate note was only ever noticed by accident.

find_duplicate_records(kind="snippet"|"note"|"task") — the same indexed
self-join, parameterised. Tasks are notes with a status, not a note_type, so
the kind split is a status predicate; mixing them would propose folding a
to-do into a write-up. find_duplicate_snippets stays as a wrapper because both
surfaces and SnippetListView consume it by name.

What differs by kind is the CURE, and the report says so in a `suggestion`
field rather than leaving the caller to guess:

  snippet  merge — lossless, the survivor keeps every call site
  note     NEVER merge. A correction pair → supersedes on the newer; state
           smeared across dated records → extract to the System's reference
           note; genuinely parallel → leave alone. Choosing needs the records
           READ, which is the agent's job — so non-snippet groups carry
           `members` with dates and any `existing_supersessions` already
           declared inside the group. A pair someone ruled on is not an open
           question.
  task     usually the same work opened twice — keep the one with the history,
           cancel the other with a pointer.

The snippet sibling filter stays snippet-only: it keys on symbol/code_sha,
which other kinds don't carry — and for them a look-alike is a finding.

Surfaces: MCP find_duplicate_records (classified into _READ_ONLY_TOOLS — the
completeness test would have caught the omission), REST /api/notes/duplicates,
and a KnowledgeView panel mirroring SnippetListView's — links only, no merge
button, because for notes the report proposes and the correction is a read-
and-decide act. The panel follows the type filter and clears when it changes,
so a note report can't linger under a task view.

Correcting the task's own premise: it claimed the snippet report had "no view
consuming it" — stale; SnippetListView has consumed it since it shipped. The
UI gap was only ever notes/tasks.

Answers the question carried from #2482: yes, the update routes on BOTH
surfaces can turn a record into a duplicate — the gate is create-time by
design. This report is the mechanism that catches it after the fact, which is
the model the operator chose.

Refs #278, #2547

frontend/src/views/KnowledgeView.vue

@@ -46,6 +46,50 @@ const sortMode = ref<"modified" | "created" | "alpha" | "type">("modified");
 const searchQuery = ref("");
 let searchDebounce: ReturnType<typeof setTimeout> | null = null;
 
+// ─── Near-duplicate report ────────────────────────────────────────────────────
+// On demand, never automatic — a corpus-wide pairwise scan, and most visits to
+// this page aren't a tidy-up (same reasoning as SnippetListView's report).
+
+interface DupMember {
+  id: number; title: string;
+  created_at?: string | null; updated_at?: string | null;
+  task_kind?: string | null;
+}
+interface DupGroup {
+  note_ids: number[];
+  members: DupMember[];
+  top_score: number;
+  existing_supersessions?: { superseder_id: number; superseded_id: number }[];
+}
+
+const dupGroups = ref<DupGroup[]>([]);
+const dupSuggestion = ref("");
+const dupLoading = ref(false);
+const dupChecked = ref(false);
+// The report follows the type filter: viewing tasks checks tasks. Anything
+// else (all / plan / process) checks notes — the kind with the most to find.
+const dupKind = computed(() => (activeType.value === "task" ? "task" : "note"));
+
+async function loadDuplicates() {
+  dupLoading.value = true;
+  try {
+    const data = await apiGet<{ groups: DupGroup[]; suggestion: string }>(
+      `/api/notes/duplicates?kind=${dupKind.value}`
+    );
+    dupGroups.value = data.groups;
+    dupSuggestion.value = data.suggestion;
+    dupChecked.value = true;
+  } catch {
+    dupChecked.value = false;
+  } finally {
+    dupLoading.value = false;
+  }
+}
+
+// A stale report is worse than none: switching the type filter changes which
+// kind the button checks, so the old kind's groups must not linger under it.
+watch(dupKind, () => { dupChecked.value = false; dupGroups.value = []; });
+
 // ─── Type counts ──────────────────────────────────────────────────────────────
 
 interface KnowledgeCounts { note: number; task: number; plan: number; process: number; total: number }
@@ -385,6 +429,51 @@ onUnmounted(() => {
             <Share2 :size="16" />
             Graph
           </button>
+          <button
+            class="btn-ghost btn-compact"
+            :disabled="dupLoading"
+            title="Find notes or tasks already recorded that closely resemble each other — the report proposes, it never changes anything"
+            @click="loadDuplicates"
+          >
+            {{ dupLoading ? "Checking…" : "Find duplicates" }}
+          </button>
+        </div>
+
+        <!-- Near-duplicate report. A proposal surface only: unlike snippets
+             (which merge losslessly), notes are never merged — the right fix is
+             supersession, extraction into a reference note, or leaving parallel
+             records alone, and choosing needs the records READ. That reading is
+             the assistant's job; this panel shows the human what exists. -->
+        <div v-if="dupChecked && !dupLoading" class="dup-panel">
+          <p v-if="!dupGroups.length" class="dup-empty">
+            No near-duplicate {{ dupKind }}s found — nothing recorded resembles
+            anything else closely enough to flag.
+          </p>
+          <template v-else>
+            <p class="dup-head">
+              {{ dupGroups.length }} possible duplicate
+              {{ dupGroups.length > 1 ? "sets" : "set" }} among your
+              {{ dupKind }}s. {{ dupSuggestion }}
+            </p>
+            <div v-for="(g, i) in dupGroups" :key="i" class="dup-group">
+              <div class="dup-members">
+                <router-link
+                  v-for="m in g.members"
+                  :key="m.id"
+                  class="dup-member"
+                  :to="dupKind === 'task' ? `/tasks/${m.id}` : `/notes/${m.id}`"
+                >
+                  #{{ m.id }} {{ m.title }}
+                </router-link>
+              </div>
+              <span class="dup-score">{{ Math.round(g.top_score * 100) }}% alike</span>
+              <span
+                v-if="g.existing_supersessions?.length"
+                class="dup-claimed"
+                title="A supersession has already been declared inside this set — it is not an open question"
+              >already ruled on</span>
+            </div>
+          </template>
         </div>
 
         <!-- Loading / empty -->
@@ -952,4 +1041,64 @@ onUnmounted(() => {
   height: 100%;
 }
 
+
+/* ── Near-duplicate report ──────────────────────────────────────────────────
+   Mirrors SnippetListView's panel so the two reports read as one feature.
+   Scoped styles can't be shared across SFCs; if a third view ever grows this
+   panel, promote the family to components.css and record it (#2464's rule:
+   two-or-more is when a recipe earns the shared sheet). */
+.dup-panel {
+  margin-bottom: 1.25rem;
+  padding: 0.85rem 1rem;
+  border: 1px solid var(--color-border);
+  border-radius: 8px;
+  background: var(--color-surface-alt);
+}
+.dup-empty,
+.dup-head {
+  margin: 0 0 0.5rem;
+  font-size: 0.85rem;
+  color: var(--color-text-muted);
+}
+.dup-empty { margin-bottom: 0; }
+.dup-group {
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+  flex-wrap: wrap;
+  padding: 0.5rem 0;
+  border-top: 1px solid var(--color-border);
+}
+.dup-members {
+  display: flex;
+  gap: 0.4rem;
+  flex-wrap: wrap;
+  flex: 1 1 20rem;
+  min-width: 0;
+}
+.dup-member {
+  font-size: 0.8rem;
+  padding: 0.1rem 0.45rem;
+  border-radius: 4px;
+  background: color-mix(in srgb, var(--color-text-muted) 12%, transparent);
+  color: var(--color-text);
+  text-decoration: none;
+  overflow-wrap: anywhere;
+}
+.dup-member:hover { background: var(--color-hover); }
+.dup-score {
+  font-size: 0.75rem;
+  color: var(--color-text-muted);
+  font-variant-numeric: tabular-nums;
+  white-space: nowrap;
+}
+/* A set someone already ruled on — quiet, not celebratory: it means "skip". */
+.dup-claimed {
+  font-size: 0.72rem;
+  color: var(--color-text-muted);
+  border: 1px solid var(--color-border);
+  border-radius: 4px;
+  padding: 0.05rem 0.4rem;
+  white-space: nowrap;
+}
 </style>