feat(linkedin-studio): trend-store staleness nudge — SessionStart B-S3 [skip-docs]

SessionStart now warns (>=7d, warn-only) when the persistent trend store's
newest capture is stale, firing ONLY when the store already holds captures
(a never-scanned user is never nagged). Neutral wording — "scan for trends"
hits trend-spotter's own trigger; no hardcoded beat (de-niche invariant).

- store.ts: newestCaptureDate() — pure max-capturedAt staleness signal (SSOT)
- cli.ts: status [--json] subcommand (count + newest + daysStale)
- session-start.mjs: trendsNewestCapture() reads trends.json as raw JSON
  (no tsx spawn at session start) + the reminder line, beside import-staleness
- tests: +3 store tests (newestCaptureDate) + hook subprocess test (3 cases:
  >=7d fires, <7d silent, absent/empty silent + no crash)
- test-runner.sh: trends floor 21->24

Verified: trends 24/24 · all hook tests 131/131 · gate 89/0/0 · real render
confirms "Trend signals are N days old. Scan for trends…".

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RigJBiRFNtFZKCz21qNbQ4

scripts/trends/tests/store.test.ts

@@ -13,6 +13,7 @@ import {
   addTrend,
   queryByTopic,
   history,
+  newestCaptureDate,
 } from "../src/store.js";
 import { SCHEMA_VERSION } from "../src/types.js";
 import type { TrendStore } from "../src/types.js";
@@ -294,4 +295,39 @@ describe("trends store", () => {
       assert.deepEqual(h.map((t) => t.title), ["Newest"]);
     });
   });
+
+  describe("newestCaptureDate (B-S3 staleness signal)", () => {
+    test("empty store → null (no captures yet → no nudge)", () => {
+      assert.equal(newestCaptureDate(emptyStore()), null);
+    });
+
+    test("single trend → its capturedAt", () => {
+      const store = addTrend(emptyStore(), {
+        title: "Solo",
+        url: "https://example.com/solo",
+        source: "tavily",
+        capturedAt: "2026-06-05",
+        topics: ["x"],
+      }).store;
+      assert.equal(newestCaptureDate(store), "2026-06-05");
+    });
+
+    test("multiple trends → the max (newest) capturedAt, regardless of insertion order", () => {
+      let store = emptyStore();
+      for (const [title, capturedAt] of [
+        ["Middle", "2026-05-01"],
+        ["Newest", "2026-06-01"],
+        ["Oldest", "2026-04-01"],
+      ] as const) {
+        store = addTrend(store, {
+          title,
+          url: `https://example.com/${title}`,
+          source: "tavily",
+          capturedAt,
+          topics: ["x"],
+        }).store;
+      }
+      assert.equal(newestCaptureDate(store), "2026-06-01");
+    });
+  });
 });