#7247 chore(docs): remove i18n and the five translated locales

Summary

• Deleted the es, fr, de, ja, and zh docs content trees (995 files) and flattened content/docs/en/** up to content/docs/**
• Removed the fumadocs i18n layer: lib/i18n.ts, i18n.json, i18n.lock, the i18n.cache gitignore entry, createI18nMiddleware in proxy.ts, and the i18n option on the source loader
• Collapsed app/[lang]/** into app/**, merging the locale layout's html/body shell into the root layout. The OpenAPI virtual pages moved off baseDir: 'en/api-reference/(generated)' — that en segment was the locale the dir parser consumed
• Stripped locale branching from the sitemap, llms.txt, llms-full.txt, the llms.mdx route, the sidebar's active-path matching, structured data, and the hreflang / alternateLocale metadata
• Simplified the docs search route to English-only: one english text-search config, unconditional vector search, and no locale-prefix filtering of sourceDocument. That filter was already inert — sourceDocument is relative to the chunker's docsPath, so it has always been an unprefixed section path (workflows/blocks/agent.mdx), and it stays byte-identical here because the pages and the docsPath default moved down the same level together
• Repointed every script, test, and the CI path filter at content/docs, and de-looped the API-reference sidebar checks over the six locales
• The execution-guide wire-shape test only covered the removed locales, so it is retargeted at the English guides (verified they already satisfy every assertion) rather than dropped

Type of Change

• Chore / maintenance

Testing

• bun run check:audits — 39/39 pass
• apps/docs build — succeeds, 666 paths prerendered, no locale prefixes
• apps/docs vitest, the OpenAPI suite (143 tests), and the netsuite docs-path test all pass
• bun run lint, check-block-registry, and apps/docs type-check clean
• Generators re-run and in sync: docs-manifest, agent-stream-docs, generate:cli-docs, docs:check, check:openapi

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)