Import the landing kit as a Hugo module instead of a variant copy

[tannevaled]

Aligns the landing on go-fleettools/landingkit v0.2.0, the way go-fileshare.github.io#24 did for the reference.

What changes

• hugo.toml imports the module; go.mod pins v0.2.0; kit.go (//go:build tools) keeps Renovate's go mod tidy from deleting the requirement; actions/setup-go runs before Hugo in deploy-pages.yml.
• Deleted: the variant styles.html (the kit's sheet without the fork-ribbon rules), theme-toggle.html, icons/book.html, icons/mark-github.html. icons/link-external.html stays as the site's own Octicon, with its OCTICONS-LICENSE.
• [params.brand] is generated by landingkit brand static/img/logo.svg and replaces the hand-derived block: base #991b1b, bright #ef4444, deep #5d1010 (was #7F1D1D), chipFg #5d1010 (was #7F1D1D), chipFgDark #f69898 (was #fca5a5), washDeep rgba(93,16,16,.06).
• The nav bar's GitHub button (with its inline SVG mark) is replaced by the fork ribbon. The hero had no source button. The ribbon points at go-pdfkit/pdfkit, the organisation's namesake library. The CTA band's "Read the source on GitHub →" still leads to the organisation.
• darkreader-lock comes from the kit's styles.html (merged into the old sheet by Tell the Dark Reader extension to stand aside (darkreader-lock) #23); it is emitted once.
• No page-specific rules: everything the page uses (the two strip lines, .tag.ready, the cards, the terminal) is in the kit, so there is no site-styles.html.

Verification

• landingkit verify go-pdfkit.github.io go-fileshare.github.io:

  PASS the <style> block equals the reference's once brand values are masked
  PASS light: 9/9 background pixels identical (#ffffff)
  PASS dark: 9/9 background pixels identical (#0b0e14)
  

• Puppeteer at 1280, 1440 and 390 (mobile emulation with setViewport), light and dark: no horizontal scroll on main or on the branch. The theme toggle is hit-tested and clicked and changes data-theme. The ribbon is hit-tested and clickable at every width.

Visual differences, section by section (main vs branch, screenshots)

• Nav: the GitHub button is gone, and the fork ribbon sits in the top-right corner. Below 1420px the nav reserves room for it, so the links and toggle sit further left.
• Hero: same layout and buttons. The gradient's far end is darker (deep #7F1D1D → #5d1010), seen in the headline accent and the primary button.
• CTA band (#eco): the same gradient change; that is the whole of its pixel difference.
• Corpus, Use it, Repositories, Where it runs, Type: no layout change. The small pixel differences are the chip/tag text (chipFg, and chipFgDark in dark mode) and the gradient on card details.
• Strip and footer: pixel-identical.

Checks

No check changes: deploy-pages (build) and docs; docs is green on main again since go-pdfkit/.github#23.

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