fix: preserve docs redirects and agent response types

[regenrek]

Summary

• add a tiny edge worker for exact legacy aliases and machine-readable docs routes
• preserve redirect query strings and restore JSON/Markdown response types
• prove human pages bypass the worker and keep the redirect inventory canonical

Root cause

Alchemy hashes _redirects and _headers but its Workers Static Assets upload path does not transmit those rules. Production therefore returned 404 for legacy aliases and application/octet-stream for Markdown and search.

Verification

• pnpm docs:typecheck
• pnpm docs:lint
• pnpm docs:verify-maintenance
• pnpm docs:verify-deployment
• pnpm docs:verify-release
• pnpm docs:verify-agent-markdown
• local Wrangler release-live: 59 pages, 38 redirects, 5 searches, query preservation and MIME/bypass markers
• local browser shell: 68 states, 20 interactions, 23 axe pages, zero serious/critical findings