fix(linkedin-studio): ship placeholder voice profile, gitignore real, sentinel detection

Wave 2 / Step 5 of the remediation plan (coupled criticals: voice-leak +
placeholder-detection).

Voice profile (the adopter-default leak):
- Ship a PII-free placeholder at authentic-voice-samples.md carrying a
  <!-- VOICE_PLACEHOLDER --> sentinel + neutral default voice principles.
- Migrate the author's real profile to gitignored authentic-voice-samples.local.md
  (already matched by *.local.md; added an explicit, commented .gitignore entry so
  the intent is unmissable). NO git-history rewrite — the historical file is
  attributed authorship, not a secret (per the plan threat model).
- Add authentic-voice-samples.template.md — a clean fill-in template for adopters.
- personalization-score.mjs: detect the sentinel (deterministic) instead of the
  unreliable `[Your Name]` heuristic, so the placeholder scores 0 voice points and
  a populated profile (sentinel removed) earns the 25.
- Both voice writers replace-not-append on the placeholder: setup.md (merge ->
  replace-if-placeholder) and onboarding.md (append -> replace-if-placeholder), so
  populating removes the sentinel; updated setup.md's stale heuristic table.

Operator decisions (deviations from plan-literal, approved this session):
- KEEP the plugin.json author name. The plan said scrub author -> neutral/org, but
  that contradicts its own LICENSE reasoning (intentional MIT attribution) and all
  5 sibling plugins keep author = the author; scrubbing only this one would create
  inconsistency for zero security gain (the name is public-by-design). The voice
  placeholder fully fixes the adopter-inheritance bug.
- Scrub the stale "January 2026 360Brew" brand from the plugin.json description and
  the "360brew" keyword (locked decision: no publishable model name/date). This is
  a Wave-1 propagation miss surfaced here because plugin.json was in Step 5's
  touch-scope.

Flagged for follow-up (NOT done here — out of Session 2 scope):
- The lint's stat-consistency grep (scripts/test-runner.sh) scans references/,
  commands/, skills/, hooks/prompts/, CLAUDE.md, README.md — but NOT
  .claude-plugin/plugin.json, which is why the 360Brew brand slipped Wave 1.
  Needs a Session-1-scoped lint extension to add plugin.json to the scan set.
- Readers (user-prompt-context.mjs, voice-guardian.md, state-update-reminder.md)
  read the tracked .md (placeholder), per the plan. The operator's real voice now
  lives in the gitignored .local.md, which nothing reads. To use it, readers + the
  voice score should prefer .local.md (matching the user-profile.local.md
  precedent). Deferred as a coherence follow-up for operator review.

Test-first: hooks/scripts/__tests__/personalization-score.test.mjs (red on the
placeholder scoring 25 under the old heuristic, green after the sentinel fix). Hook
suite 62/62, structural lint 0 failed.

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

plugins/linkedin-studio/.claude-plugin/plugin.json

@@ -1,11 +1,11 @@
 {
   "name": "linkedin-studio",
   "version": "3.1.0",
-  "description": "LinkedIn Studio — full-spectrum LinkedIn content engine: feed posts, carousels, video scripts, and long-form newsletter editions, with the January 2026 360Brew algorithm baked in. v3.1.0 (Endring 9) adds a cold adversarial review package to `/linkedin:newsletter` — Step 6.5 + the standalone `/linkedin:headless-review` command run three new headless archetypes (content-reviewer, language-reviewer, fact-reviewer) plus the persona reviewer with NO drafting-session context — a `/linkedin:pivot` command that re-opens cleared gates after a late change, and per-artifact personas (one or more readers configurable per edition). v3.0.0 renamed the plugin (was `linkedin-thought-leadership`): slug, agent namespace, and state-file path are `linkedin-studio`; the `/linkedin:*` commands are unchanged.",
+  "description": "LinkedIn Studio — full-spectrum LinkedIn content engine: feed posts, carousels, video scripts, and long-form newsletter editions, with the 2026 relevance-ranking model baked in. v3.1.0 (Endring 9) adds a cold adversarial review package to `/linkedin:newsletter` — Step 6.5 + the standalone `/linkedin:headless-review` command run three new headless archetypes (content-reviewer, language-reviewer, fact-reviewer) plus the persona reviewer with NO drafting-session context — a `/linkedin:pivot` command that re-opens cleared gates after a late change, and per-artifact personas (one or more readers configurable per edition). v3.0.0 renamed the plugin (was `linkedin-thought-leadership`): slug, agent namespace, and state-file path are `linkedin-studio`; the `/linkedin:*` commands are unchanged.",
   "author": {
     "name": "Kjell Tore Guttormsen"
   },
   "license": "MIT",
   "repository": "https://git.fromaitochitta.com/open/ktg-plugin-marketplace",
-  "keywords": ["linkedin", "content-creation", "newsletter", "analytics", "360brew"]
+  "keywords": ["linkedin", "content-creation", "newsletter", "analytics", "relevance-ranking"]
 }