Portal redirects to apply when promoting the XP 8.1 docs to stable

[sigdestad]

When versions.json promotes the 8.1 documentation (currently master, published as "next") to stable, the developer portal needs redirects for the pages that moved. The fundamentals paths have been public since May 2026.

From (docs/code/stable/...)	To
fundamentals	runtime
fundamentals/runtime	runtime
fundamentals/apps	apps
fundamentals/typescript	tooling/typescript
fundamentals/types	libraries/types
fundamentals/engines, functions, main, globals, events, i18n, java-bridge	runtime/<same>
runtime/apps	apps
upgrade/8-1	upgrade

Two dependencies:

• Promote together with doc-platform (Portal redirects to apply when promoting the XP 8.1 docs to stable doc-platform#603). The 8.1 platform release notes link into pages that exist only in the 8.1 dev kit docs.
• xp-app-upgrader: read upgrade guides from the developer portal agent-toolkit#12 changes the xp-app-upgrader skill to fetch docs/code/stable/upgrade/xp7 and docs/platform/stable/upgrade/xp7 from the portal. Merge it only once those pages are live on stable.