feat(linkedin-studio): video quality gate (captions + aspect guidance, drop 9:16 mandate)

Resolve the video-advice contradiction on the two surfaces this step owns
(research/03 D1-D3):

- commands/video.md: rename the "3-Second Test" to a "Muted-Autoplay Test"
  (front-load value for ~85% muted viewing; the "three-second hook" is
  cross-platform folklore, not a LinkedIn signal); make captions the
  enforceable spec (SRT or native auto-captions, indexed for search);
  aspect ratio as guidance (4:5/1:1 preferred for broad distribution,
  9:16 opt-in for the vertical video tab, crops to 1:1 on desktop); add
  MP4-default + warn-only MOV/AVI + official upload limits to the
  pre-recording reminder.
- references/linkedin-formats.md: reframe the 9:16 "distribution boost"
  as uncorroborated heuristic; 4:5 "deprioritized" -> 4:5/1:1 preferred;
  drop the "3-second hook"; MP4 safe-default + warn-only MOV/AVI; replace
  "good video rewarded more than ever" with the honest "per-video reach
  declining; documents/carousels out-engage video."

algorithm-signals-reference.md (the canonical magnitude source) was
already reconciled in Phase 0 (line 56 reads "declining ... 4:5/1:1
preferred, captions are the enforceable spec") and carries no boost copy,
so it is intentionally untouched here — and it is not in this step's
manifest.

Verify: grep 'must be 9:16|9:16 (1080|3-second hook' video.md
linkedin-formats.md -> none; 'captions' video.md -> 5; 'deprioritized'
linkedin-formats.md -> none; structure lint 61/61.

KNOWN RESIDUAL (flagged for follow-up, NOT in this step's scope):
- references/linkedin-growth-playbook-2025-2026.md:216 still carries a
  "9:16 ... distribution boost" line. That file is owned by Step 17, but
  Step 17's changes are newsletter-distribution — the video line would
  fall through. Fold this into Step 17 or the review gate.
- references/video-strategy-guide.md:300 still says "the 3-second test
  determines 70% of retention." That file is owned by NO plan step
  (orphaned) — needs a home. Surface at session end for an operator
  decision.

Plan Step 15 (Wave 4 S2). Counts unchanged.
[skip-docs]: tre-doc + version bump deferred to Step 21 per remediation plan.

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

plugins/linkedin-studio/commands/video.md

@@ -94,8 +94,9 @@ Delegate script generation to the `video-scripter` agent — invoke it via `Task
 
 Before presenting, verify the script passes the video quality gate:
 
-**The 3-Second Test:**
-- [ ] Hook grabs attention in first 3 seconds (8 words or fewer)
+**The Muted-Autoplay Test:**
+- [ ] Opening front-loads value for muted autoplay — ~85% watch without sound (the "three-second hook" is cross-platform folklore, not a LinkedIn signal; LinkedIn's only official "3 seconds" is the minimum video length)
+- [ ] First line reads on-screen as text/caption, not only when spoken
 - [ ] No "Hey everyone" or "So today I'm going to talk about..."
 
 **Natural Speech Test:**
@@ -114,7 +115,7 @@ Before presenting, verify the script passes the video quality gate:
 - [ ] Energy peaks at hook and takeaway
 
 **Completeness Test:**
-- [ ] Captions written and synced
+- [ ] Captions written and synced — the enforceable spec (~80–85% watch muted; caption text is indexed for search/discovery). Accept SRT upload OR LinkedIn native auto-captions
 - [ ] Post caption (200-400 chars) included
 - [ ] Thumbnail suggestion included
 - [ ] First comment pre-written
@@ -202,7 +203,8 @@ Before you record:
 - [ ] Read the script aloud once (practice run)
 - [ ] Set up lighting (natural light facing window, or ring light)
 - [ ] Check audio (lavalier mic or quiet room)
-- [ ] Vertical format: 9:16 (1080×1920) for LinkedIn vertical feed (3-4x watch duration vs landscape)
+- [ ] Aspect ratio: 4:5 (1080×1350) or 1:1 for broad feed distribution; reserve 9:16 for the opt-in vertical video tab (it crops to 1:1 on desktop)
+- [ ] Export as MP4 (H.264) — the safe default; keep within LinkedIn limits (≤10 min mobile / 15 min desktop, ≤5GB). MOV/AVI is warn-only — re-encode to MP4 if unsure
 - [ ] Clean background
 - [ ] Have captions tool ready (CapCut, Descript, or Kapwing)
 - [ ] First comment ready to paste immediately after posting