fix(desktop): exclude archived channels from workstream board discovery

sq agents review flagged that the prefix-only discovery filter let an
archived loganj-ws-* channel keep appearing as an active board card.
Every other active-channel surface in this codebase (sidebar, search,
channel browser, agent autocomplete) excludes archivedAt !== null, so
apply the same rule here.

The review's second finding — one relay canvas query per rendered card
via useCanvasQuery — is the contract's specified design ("per matching
channel canvas fetch via existing canvas query/API paths") for this
bullet; batching/limiting canvas fetches is left for a later slice
rather than implemented as an unscoped addition here.

Signed-off-by: loganj <loganj@squareup.com>
Co-authored-by: Claude Code <noreply@anthropic.com>

desktop/src/features/workstream-board/lib/discoverWorkstreamChannels.test.mjs

@@ -18,7 +18,7 @@ function buildChannel(overrides) {
     memberCount: overrides.memberPubkeys?.length ?? 0,
     memberPubkeys: overrides.memberPubkeys ?? [],
     lastMessageAt: null,
-    archivedAt: null,
+    archivedAt: overrides.archivedAt ?? null,
     participants: [],
     participantPubkeys: [],
     isMember: overrides.isMember ?? false,
@@ -103,6 +103,23 @@ test("applies no creator/membership filter — every matching name is included r
   ]);
 });
 
+test("excludes an archived channel even when its name matches the prefix", () => {
+  const channels = [
+    buildChannel({
+      id: "1",
+      name: "loganj-ws-done",
+      archivedAt: "2026-01-01T00:00:00Z",
+    }),
+    buildChannel({ id: "2", name: "loganj-ws-active" }),
+  ];
+
+  const result = filterWorkstreamChannels(channels);
+  assert.deepEqual(
+    result.map((c) => c.id),
+    ["2"],
+  );
+});
+
 test("returns an empty array when nothing matches", () => {
   const channels = [
     buildChannel({ id: "1", name: "general" }),

desktop/src/features/workstream-board/lib/discoverWorkstreamChannels.ts

@@ -2,15 +2,18 @@ import type { Channel } from "@/shared/api/types";
 
 /**
  * Channels whose name starts with this prefix are discovered as workstream
- * board entries. There is no creator/ownership filter — any visible channel
- * matching the prefix is included, regardless of who created or joined it.
+ * board entries. There is no creator/ownership filter — any visible,
+ * non-archived channel matching the prefix is included, regardless of who
+ * created or joined it.
  */
 export const WORKSTREAM_CHANNEL_PREFIX = "loganj-ws-";
 
 export function filterWorkstreamChannels(
   channels: readonly Channel[],
 ): Channel[] {
-  return channels.filter((channel) =>
-    channel.name.startsWith(WORKSTREAM_CHANNEL_PREFIX),
+  return channels.filter(
+    (channel) =>
+      channel.name.startsWith(WORKSTREAM_CHANNEL_PREFIX) &&
+      channel.archivedAt === null,
   );
 }