[JJLiebig]

Change

Add repo-owned coding guidance for the TypeScript @nextide/ui library and its playground consumer, with a pointer outside the managed AGENTS block. Copy Meta's typescript-application.md exactly from 554cc0f4284f3aac00feee5ff42771f232739ed7.

The local index preserves Prettier, separate compiler checks, untyped Vite+ lint and the existing React Hooks/accessibility rules. No Python application or helper files are present. No tooling migration or runtime change.

Validation

• Read the affected instructions and current source/tool ownership.
• Relative Markdown targets, exact shared-file byte/blob parity, unchanged existing AGENTS content/managed block and narrow docs-only scope checked.
• git diff --check passed.
• Commit hook passed: whole-tree Prettier, Vite+ lint, supply-chain watchlist and all four TypeScript projects.
• CI explicitly waived by the maintainer for this rollout; no CI pass is claimed. Runtime suites and new tests skipped for prose-only changes.
• Parent owns final independent rollout review and merge; this PR does not deploy.