Typecheck test/ and scripts/, not just src/

[dangrondahl]

Description

tsconfig.json has include: ["src"] and excludes test, so npm run build typechecks only src/. Vitest doesn't typecheck either — it strips types and runs. That means neither test/ nor scripts/ is ever seen by the compiler.

Surfaced during the review of #28 (@types/node 22 → 26), where it materially weakened the evidence: a green CI run says nothing about whether a types bump broke test/client/kosli-client.test.ts (which asserts on FormData/Blob) or scripts/generate-catalog.ts (which imports node:fs and node:url). Those are exactly the files a @types/node major is most likely to affect, and they're outside the compiler's view.

This matters more now that #38 pins @types/node to the Node floor: the pin is only enforceable if tsc actually sees the code that would break.

Proposal

• Add tsconfig.typecheck.json extending tsconfig.json, with noEmit: true and include: ["src", "test", "scripts"].
• Add a typecheck script: tsc --noEmit -p tsconfig.typecheck.json.
• Wire it into ci.yml as its own step, so a failure is distinguishable from a build or test failure.
• Fix whatever it surfaces. Keep strict: true — don't loosen the config to make errors go away.

Keep the existing tsconfig.json as the emit config so npm run build still produces dist/ from src/ alone.

Tasks

• Add tsconfig.typecheck.json
• Add the typecheck npm script
• Add a CI step
• Fix surfaced type errors