feat(scripts): migrate retext-runner to type-checked TypeScript #3174

Issue Description

Convert the eval text-moderation runner scripts/evals/Modules/retext-runner.mjs to strictly typed TypeScript as retext-runner.mts, and enforce the types in local validation and PR CI.

Node 24, the version pinned in CI, the devcontainer, and Copilot setup, runs .mts files directly through built-in type stripping, so no build step or generated output is needed. .mts is used instead of .ts because the root package.json has no "type": "module". A .ts file would print a MODULE_TYPELESS_PACKAGE_JSON warning to stderr on every run.

Scope:

• Rename the runner to .mts with strict, erasable-only types. Its stdin/stdout/exit-code contract and output must stay byte-identical.
• Add exact-pinned typescript and @types/node dev dependencies, a root tsconfig.json, and an npm run lint:ts (tsc --noEmit) check, and add lint:ts to validate:local.
• Run lint:ts on every PR through a new reusable typescript-check.yml workflow that is part of the PR validation gate.
• Point Test-EvalSpecText.ps1 at the new file.
• Teach Test-CopyrightHeaders.ps1 to scan .mts with // comments. Today .mts is skipped.
• Update the contributor docs that name the runner path or list validation commands.

Additional Context

• While prototyping, the type check found a dead fallback in the current runner: message.position does not exist on vfile-message v4 messages. The conversion removes it without changing behavior.
• Other .mjs files, a root "type": "module", and .cts support are out of scope.
• Known follow-up, not part of this issue: the root spell-check glob does not scan .mjs, .mts, or .cjs sources.