feat(linkedin-studio): parameterize series path + de-brand render output [skip-docs]

Wave 3 / Step 8 of the remediation plan (Phase 1 — usable by a non-author).

The flagship long-form engine shipped bespoke-as-general: a maintainer-private
absolute series path and a hardcoded 'Maskinrommet' brand in the renderers. Both
are now generalized so a non-author can run the pipeline without inheriting the
author's filesystem or publication identity.

- commands/newsletter.md + config/edition-state.template.json: series-root default
  /Users/ktg/repos/maskinrommet/serier -> $HOME/linkedin-series; reconciled the
  prose that called the maskinrommet folder 'the default' so it no longer
  contradicts the new neutral default. LTL_SERIES_ROOT override contract preserved.
- render/build-linkedin.mjs + render/build-carousel.mjs: brand is now an LTL_BRAND
  env-var (empty default -> generic). Samle-post title, carousel footer brand-span,
  and cover-eyebrow fallback are de-branded; empty brand renders clean chrome. The
  operator sets LTL_BRAND=Maskinrommet in their own env to re-brand (same pattern
  as LTL_SERIES_ROOT).
- config/image-credit-caption.template.md: 'Maskinrommet-/serie-badge' -> generic
  'serie-badge'.

Out of scope (Step 9): the residual 'Maskinrommet skrivekontrakt §C2/§A'
references in newsletter.md are the writing-contract generalization, handled next.

[skip-docs]: three-doc + version reconciliation is Step 21 (pre-review-gate, per
plan: 'LAST so it captures everything'). These intermediate Wave commits are NOT
pushed before the /trekreview gate, so the three-doc obligation (which governs
pushed changes) is satisfied at Step 21, not per local checkpoint commit.

Verify: grep -rIn '/Users/ktg' config/ commands/ render/ (excl .local) -> no
matches; grep -rn 'Maskinrommet' render/ -> no matches (de-branded); node --check
on both render scripts -> OK; LTL_SERIES_ROOT still present in newsletter.md.

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

plugins/linkedin-studio/config/image-credit-caption.template.md

@@ -59,5 +59,5 @@ som *også* legger en feed-cover trenger likevel en rad over.
 
 ## Samle-post
 
-Ev. Maskinrommet-/serie-badge (egen asset) → ingen credit. Lenken til serien ligger i
+Ev. serie-badge (egen asset) → ingen credit. Lenken til serien ligger i
 første kommentar, ikke i bildet.