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/commands/onboarding.md

@@ -8,6 +8,7 @@ description: |
   "just installed", "how do I start", "walk me through", "linkedin onboarding".
 allowed-tools:
   - Read
+  - Write
   - Bash
   - AskUserQuestion
 ---