feat(linkedin-studio): S16 — optional manual saves in analytics + close deferred onboarding Write MAJOR

Lifts the original v4.0.0 Non-Goal: an optional, manually-entered `saves`
metric through the analytics layer, built location-agnostic (option c) so
UI-brief §9b/M0 relocates the data dir in one place later.

- types: PostMetrics.saves? + Weekly/Monthly summary.totalSaves? (optional);
  new RankableMetric type for the always-numeric index-access whitelist
- parser: dedicated parseOptionalCount() — blank/non-numeric/negative -> undefined
  ("unknown != 0"), genuine 0 kept; saves NOT folded into engagementRate
- reports: totalSaves set only when >=1 post carries saves (backward-compat)
- cli: saves surfaced in import summary + weekly/monthly totals + per-post
- S16-pre: onboarding.md allowed-tools gains Write (closes S15-deferred MAJOR)
- docs (three-doc rule): plugin README boundary + analytics README + root README
  + plugin CLAUDE.md + CHANGELOG; dwell stays explicitly unmeasurable

Independent /trekreview: brief-conformance 0 findings; code-correctness 2 MAJOR
(own lockstep misses) FIXED in-session (parseOptionalCount + edge tests). Gate:
tsc clean, analytics 116/116, lint 74/0/0, hooks 98/98. Within-v4.1.0 refinement
(no surface/count/version change).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugins/linkedin-studio/scripts/analytics/tests/monthly.test.ts

@@ -83,6 +83,27 @@ describe("generateMonthlyReport", () => {
     assert.equal(report.summary.avgImpressionsPerPost, 2000);
   });
 
+  test("sums saves into totalSaves when posts carry manual saves data", () => {
+    const withSaves = (p: PostAnalytics, saves: number): PostAnalytics => ({
+      ...p,
+      metrics: { ...p.metrics, saves },
+    });
+    const posts: PostAnalytics[] = [
+      withSaves(createPost("2026-03-03", 1000, 3.0), 8),
+      createPost("2026-03-05", 2000, 4.0), // no saves — partial coverage
+      withSaves(createPost("2026-03-10", 1500, 3.5), 13),
+    ];
+    const root = setupTestRoot(posts);
+    const report = generateMonthlyReport(root, "2026-03");
+    assert.equal(report.summary.totalSaves, 21, "totalSaves should sum 8 + 13");
+  });
+
+  test("leaves totalSaves undefined for saves-free months (backward-compat)", () => {
+    const root = setupTestRoot(marchPosts);
+    const report = generateMonthlyReport(root, "2026-03");
+    assert.equal(report.summary.totalSaves, undefined);
+  });
+
   test("generates weekly breakdown within month", () => {
     const root = setupTestRoot(marchPosts);
     const report = generateMonthlyReport(root, "2026-03");