fix(linkedin-studio): close v4.0.0 audit review findings (S8)

Close the 5 findings from the S7 /trekreview release gate (review.md, verdict BLOCK):

- BLOCKER: comment-multiplier "5x" reconciled to the canonical order-only framing
  (no fixed multiplier) in agents/engagement-coach.md, linkedin-growth-playbook,
  linkedin-formats.md — per algorithm-signals-reference.md ("do not quote a comment
  multiplier").
- BLOCKER: carousel rate "6.60%/6.6% (highest)" reconciled to "~7% top organic
  format" in linkedin-formats.md:42 (was self-contradicting :50) and
  assets/templates/carousel-templates.md.
- Lint hardening: test-runner.sh STALE_STATS now matches 6.60% + the 5x comment
  folklore and scans agents/ + assets/templates/ — the grep that defines the
  Phase-0 criterion now catches both BLOCKERs.
- MAJOR: onboarding.md command count 26 -> 27.
- MAJOR: add section-append-branch (production-path) tests for recordFirstHourPlan
  + recordOutreachContact against a template-layout fixture.
- MINOR: move date-scalar changes.push inside the write branch in state-updater.mjs.

Verify: node --test hooks/scripts/__tests__/*.test.mjs -> 92/92; bash
scripts/test-runner.sh -> 66/0/0. NO push until /trekreview re-confirms ALLOW/WARN.

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

plugins/linkedin-studio/references/linkedin-formats.md

@@ -39,7 +39,7 @@ Choosing the right format isn't just about engagement rates—it's about underst
 
 ### Engagement Rates by Format (With Strategic Context)
 
-**1. Multi-image carousels: 6.60% engagement rate (highest)**
+**1. Documents / carousels (PDF posts): top organic format (~7%; see `references/algorithm-signals-reference.md`)**
    - 6-10 slides optimal
    - 100-150 characters per slide
    - Caption: 300-500 characters
@@ -171,7 +171,7 @@ The first hour after posting determines 70% of your post's total reach. See the
 2. **Shares** (High signal - people want to show others)
 3. **Comments 15+ words** (High signal - 2x impact vs short comments)
 4. **Comments <15 words** (Medium signal)
-5. **Reactions** (Lower signal - 5x less valuable than comments)
+5. **Reactions** (Lower signal - baseline engagement unit; see `references/algorithm-signals-reference.md`)
 
 **AI-generated generic comments reduce reach by 30% and engagement by 55%**