Phase 2: add ts-frontend and ts-backend stacks to core:setup-repo

[jonnyspicer]

Extend the core:setup-repo skill (Phase 1 shipped in PR #1) with the TypeScript stacks, per the design decisions locked during Phase 1.

Scope

Two new stack variants under the plugin's bundled assets:

• ts-frontend — TypeScript + Vite + Vitest + ESLint 9 (flat) + typescript-eslint + React 18.
• ts-backend — TypeScript + tsx + Vitest + ESLint 9 (flat) + typescript-eslint.
• npm as the package manager (matches every existing TS repo).

Work

• Add assets/stack/ts-frontend/ and assets/stack/ts-backend/ (package.json, tsconfig.json, eslint.config.js, vitest.config.ts, node gitignore).
• Add assets/ci/ci-node.yml (eslint + tsc + vitest).
• Extend scripts/scaffold-repo.sh: accept ts-frontend/ts-backend stacks, copy the right assets, wire ci-node.yml.
• Extend SKILL.md stack detection/choice (package.json/tsconfig.json → ts; ask frontend vs backend) and the install step (npm).
• Extend the test suites to cover the new stacks and CI asset.
• Bump the core plugin version (update signal).

Out of scope

Phase 3 (pre-commit, C++/CMake for blah2-arm/retina-spectrum) — separate.

Context

Real demand: the Tower-Finder frontends (Vite/React/Vitest/ESLint 9) and the adsb2dd Node backend. The frontend pattern is the established one; ts-backend replaces the ad-hoc plain-JS pattern.