fix: add localized legacy redirects

[steipete]

What Problem This Solves

R2 Pages fails its internal route audit while localized plugin inventories still link a recently retired route such as /ar/plugins/reference/codex-supervisor. English redirects exist, but the static builder did not emit locale-prefixed redirect files, so translation lag blocked every docs publish.

Why This Change Was Made

Generate locale-prefixed variants for every configured redirect instead of weakening the route audit or restoring retired plugin pages. Redirects use the localized destination when that page exists and fall back to the canonical English destination until translation catches up. Existing authored pages remain untouched.

User Impact

Docs publishes can proceed while localized pages lag behind English source changes. Stale localized links continue to resolve, and users stay in their locale whenever the translated destination exists.

Evidence

• make docs-check — passed; rendered 27,898 pages, prepared 78,652 R2 objects, indexed 21 languages, and passed the full internal route audit.
• DOCS_SITE_BASE_PATH=/docs npm run docs:build:shell — generated both base-path and locale-prefixed redirect variants.
• Direct artifact checks proved /ar/plugins/reference/codex-supervisor falls back to /plugins/codex-supervision, while /ar/session redirects to /ar/concepts/session.
• Codex autoreview — clean after fixing its accepted base-path smoke finding.

[steipete]

Land-ready verification:

• Local: make docs-check — passed in 134s; rendered 27,898 pages, prepared 78,652 R2 objects, indexed 21 languages, and passed the full route smoke.
• Base-path proof: DOCS_SITE_BASE_PATH=/docs npm run docs:build:shell generated locale/base-path redirect variants. Direct artifact checks confirmed the stale Arabic Codex Supervisor route falls back to the canonical English supervision page, while existing localized redirect targets stay localized.
• Review: Codex autoreview accepted one base-path smoke finding; after the fix, the second review returned no accepted/actionable findings.
• Exact head: ae04e0c9e59a99efb2e3c295ee089d8bd53e83fa.
• CI: Docs Code CI run 29168539343 passed; CodeQL run 29168539378 passed both Actions and JavaScript/TypeScript analyses.

Known gap: the active translation workflow has not yet regenerated the localized Codex supervision pages. This change intentionally keeps stale localized links valid until that generated update lands; it does not weaken the internal route audit.