chore: migrate workspace to TypeScript 7 beta via tsgo

[tkstang]

Purpose

Switch the workspace to the current TypeScript 7 beta/native-preview transition path by using @typescript/native-preview for compiler entry points, keeping the TypeScript 6 compatibility alias for tooling that still imports typescript, and removing TS 7-incompatible baseUrl usage.

Changes

• Replace package build/type-check/watch scripts that used tsc with tsgo
• Add @typescript/native-preview@beta and alias typescript to @typescript/typescript6
• Remove TS 7-incompatible baseUrl usage and add docs app type-check generation step
• Add CSS declaration shims required by the new explicit docs-app tsgo --noEmit check
• Update shipped Fumadocs scaffold templates and scaffold tests for the TS 7 transition path
• Update contributor documentation for the TS 7/tsgo workflow

Testing

• pnpm build
• pnpm type-check
• pnpm lint
• pnpm test
• pnpm --filter oat-docs type-check
• pnpm build:docs
• pnpm release:validate
• pnpm --filter @open-agent-toolkit/cli test
• pnpm --filter @open-agent-toolkit/cli type-check

Notes

Two migration-specific follow-ups were needed during validation:

1. The new explicit docs-app tsgo --noEmit check surfaced a missing CSS module declaration for the Next app's globals.css side-effect import, so a minimal global.d.ts shim was added both in-repo and in the shipped Fumadocs template.
2. The scaffold tests still expected the old baseUrl template contract and TypeScript 5 pin; those expectations were updated to the TS 7-compatible template shape.

An early parallel validation attempt also collided on packages/cli/assets because bundle-assets.sh rewrites that directory; sequential validation avoided that false negative.