Add a webjs typecheck command (tsc --noEmit wrapper)

[vivek7405]

Problem

webjs ships erasable TypeScript as a first-class authoring mode (tsconfig strict+noEmit+erasableSyntaxOnly, @webjsdev/ts-plugin) but there is no command to run the type checker. The runtime only catches strip-time failures (non-erasable syntax) with a 500; genuine type errors are never surfaced by any webjs command or in CI, and webjs check is correctness-only and explicitly does not type-check. So a webjs app has no first-party is-my-TypeScript-valid gate, and the scaffold ships no script to start from.

Design / approach

A thin command that shells out to the project's own tsc --noEmit and a scaffolded npm script. The framework runs the standard compiler, it does not embed one.

Web-standards fit: Delegates to the standard tsc the project already has; the command is a few lines of process spawning.

Prior art: Next.js runs tsc in next build; Remix wires a typecheck: tsc script into every scaffold.

Acceptance criteria

• webjs typecheck runs tsc --noEmit over the app and exits non-zero on type errors
• Scaffolded package.json includes a typecheck script wired to the command
• It degrades gracefully with a clear message when typescript is not installed
• Documented in the AGENTS.md CLI reference and the scaffold's CONVENTIONS.md test/CI section
• Tests cover the new behaviour at the applicable layer(s)
• Docs / AGENTS.md / CONVENTIONS.md updated if the public surface changed

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_{Filed from the production-readiness audit (webjs vs Next.js / Remix / Rails / Turbo / Lit). Theme: dx-cli. Priority: P1. Kept to webjs identity: no-build, progressive enhancement, web-components-first, AI-first, batteries-included, close to web standards.}