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/test-runner.sh

@@ -683,7 +683,7 @@ if [ -x "$TR_DIR/node_modules/.bin/tsx" ]; then
   TR_OUT=$( set +e; (cd "$TR_DIR" && npm test) 2>&1; echo "TR_EXIT:$?" )
   TR_EXIT=$(echo "$TR_OUT" | grep -oE 'TR_EXIT:[0-9]+' | grep -oE '[0-9]+' | head -1)
   TR_TESTS=$(echo "$TR_OUT" | grep -oE 'tests [0-9]+' | grep -oE '[0-9]+' | tail -1)
-  TRENDS_TESTS_FLOOR=21
+  TRENDS_TESTS_FLOOR=24  # B-S3: +3 newestCaptureDate tests (staleness signal)
   if [ "$TR_EXIT" = "0" ] && [ -n "$TR_TESTS" ] && [ "$TR_TESTS" -ge "$TRENDS_TESTS_FLOOR" ]; then
     pass "trends-store suite green: $TR_TESTS tests pass (floor $TRENDS_TESTS_FLOOR)"
   else