Unified CTA tracking partials, matching Nuxt's PostHog data

[Yndira-E]

Description

This is a continuation of #5626, and extends the same CTA tracking unification to Nuxt:

• Genuine single source of truth: nuxt/lib/cta-destinations.ts now imports src/_data/ctaDestinations.json and site.json instead of hand-duplicating the values.
• Build-time validation: cta-button.njk validates variant/color and fails with a descriptive error on an invalid value, instead of silently emitting a nonexistent CSS class.
• Remaining hand-written CTAs migrated to the shared macros: dashboard.njk, plc.njk, the deployment cards in node-red/index.njk (which also replaces a fragile URL-string comparison dispatch with an explicit cta field).
• Dead frontmatter cleanup: removed buttonText/buttonLink/primaryCta/secondaryCta fields left unused after those migrations.
• Consistent position naming site-wide, in both frameworks: fixed values that duplicated the page name/slug (opcua-hero, customer-story, etc.) or used meaningless placeholders (primary/secondary) so they describe actual on-page placement instead (hero, bottom-cta-card, right-column-summary, deployment-cards, certification).
• Documented in CLAUDE.md, as a gotcha, the temptation to prefix position with the page that embeds a component shared across routes (known, left-as-is case: AgentSetupTabs.vue).

Related Issue(s)

Checklist

• I have read the contribution guidelines
• I have considered the performance impact of these changes
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
• For blog PRs, an Art Request has been created (instructions)

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