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/README.md

@@ -437,9 +437,13 @@ profile**. As of 2026-05:
   Community Management API app + a verified organization + a Page). It is **not
   self-serve** for a solo personal profile, so the practical floor is the **CSV
   export** you drop into `/linkedin:import`. Per-post **saves** are visible in your
-  **native** LinkedIn post analytics (count-only, since ~Sept 2025), but there is
-  no self-serve API to pull them — this tool does not auto-track them; read them in
-  LinkedIn directly.
+  **native** LinkedIn post analytics (count-only, since ~Sept 2025) but are absent
+  from the CSV export, and there is no self-serve API to pull them. The tool does
+  **not** auto-track saves — but you can record them **manually**: add a `Saves`
+  column to the CSV with the counts you read off LinkedIn, and `/linkedin:import`
+  ingests them and surfaces them in the weekly/monthly reports (omit the column and
+  saves simply stays unknown — never counted as 0, never folded into engagement
+  rate).
 - **Auto-publish** — technically **possible** self-serve via the `w_member_social`
   scope, so this is **not** an API limitation. LinkedIn Studio **deliberately does
   not** post for you: the OAuth/token overhead plus LinkedIn's terms on automated