Fold app/ onto TypeScript 7 once typescript-eslint supports the TS 7.1 API

[KvFxKaido]

Follow-up to #1044, which moved cli/ and mcp/github-server onto TypeScript 7.0 RC (typescript@rc) but held app/ back on the tsgo + TS 6 split.

Why app/ was held

TypeScript 7.0 only exposes the ./unstable/* programmatic API. typescript-eslint depends on the legacy programmatic API, which Microsoft says lands in TS 7.1 (several months out). Moving app/ to typescript@rc now breaks ESLint with ERR_PACKAGE_PATH_NOT_EXPORTED. So app/ still:

• typechecks via tsgo (@typescript/native-preview) — already the TS 7 checker, so no type-coverage gap today
• keeps typescript@^6.0.3 for ESLint (typescript-eslint) and its (unused) tsc emit

What to do when TS 7.1 ships with eslint support

Once typescript-eslint declares support for the TS 7.1 programmatic API:

1. In app/package.json: replace typescript@^6.0.3 with typescript@rc/latest and drop @typescript/native-preview.
2. Swap app/'s typecheck script from tsgo back to tsc (tsc --noEmit -p tsconfig.app.json && tsc --noEmit -p tsconfig.node.json).
3. Now that all three packages use tsc, the typecheck:all umbrella runs a single compiler — consider whether the tsgo/@typescript/native-preview fallback wording in the docs is still needed.
4. Verify: cd app && npm run lint && npm run typecheck, plus the root typecheck:all and the cli/mcp builds.

Docs to update in the same change

The split toolchain is described in ARCHITECTURE.md, CONTRIBUTING.md, CLAUDE.md, and REVIEW.md — collapse those back to a single TS 7 story once app/ folds in.

Signals to watch

• typescript-eslint release notes / its SUPPORTED_TYPESCRIPT_VERSIONS range advertising TS 7.x
• TypeScript 7.1 shipping the stable programmatic API (also unblocks ts-morph, custom transformers, etc.)