ci: add `tsc --noEmit` step to `npm test`

[samfoy]

Surfaced during review of #6: a missing import in subagent.ts was nearly shipped and caught only because reviewers manually ran tsc. The current npm test runs vitest but no type-check, so tests that import from peer-dep-free modules (e.g. subagent-diagnostics.ts) don't exercise the main extension's type graph.

Goal
Add a type-check step that fails the build on type errors in src/, without requiring pi peer deps at runtime install time.

Why it's not trivial
The project intentionally keeps @mariozechner/pi-coding-agent, @mariozechner/pi-ai, and @sinclair/typebox as peer deps so consumers resolve a single copy. tsc still needs to find those types during the check, so they need to be present as dev deps (or the tsconfig needs to reference them via paths / a types-only package).

Proposed approach

1. Add a minimal tsconfig.json covering src/ with noEmit: true, strict: true, moduleResolution: bundler.
2. Add peer deps as dev deps so pnpm install resolves types for the check.
3. Add "typecheck": "tsc --noEmit" to package.json scripts.
4. Wire typecheck into npm test (runs before vitest).
5. Verify with a deliberately broken import (revert after) that the check actually fails.

Non-goals

• Emitting .d.ts or compiled JS — the extension ships .ts direct via pi's tsx loader.
• Adopting stricter lint rules in the same PR — keep it to the type-check introduction.