chore(scripts): type-check the qa scripts

[code-yeongyu]

Summary

scripts/qa/*.ts were outside every tsconfig, so type errors there never failed CI.

Before this change, tsc --noEmit -p scripts/tsconfig.json reported 7 errors (4× TS7016 on untyped local .mjs imports in scripts/qa/omp-item1.ts and scripts/qa/omp-item2.ts, plus 3 ambient-module misses from compiling the qa graph without coding-agent *.d.ts files). After: 0.

No compiler options were loosened (strict remains on; no noImplicitAny: false; no extra skipLibCheck).

Changes

• Add scripts/tsconfig.json extending the root config, including qa/**/*.ts.
• Include scripts/qa/**/*.ts in root tsconfig.json so existing tsc --noEmit covers them.
• Run tsc --noEmit -p scripts/tsconfig.json from the root check script after the root program.
• Add .d.mts shims for the four local .mjs modules the qa runners import.

Changelog

Package CHANGELOG.md is not updated: this is not runtime source (packages/*/src), which the changelog gate exempts. changes.md (root) and scripts/changes.md cover the production paths.

Risks

A pre-existing intermittent flake in test/experimental-remote-runtime.test.ts (#1676) may fail an unrelated coding-agent CI shard; do not treat that as this PR.

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Summary by cubic

Brings scripts/qa/*.ts under tsc so type errors there fail CI instead of being silently ignored.

• Adds scripts/qa/**/*.ts to the root tsconfig.json and a new scripts/tsconfig.json covering qa/**/*.ts plus packages/**/*.d.ts.
• Runs tsc --noEmit -p scripts/tsconfig.json from the root check script after the root program.
• Adds .d.mts shims for the four local .mjs modules the qa runners import.
• Resolves the 7 pre-existing errors (4× TS7016 and 3 missing ambient-module declarations) without loosening compiler options.
• A pre-existing flake in test/experimental-remote-runtime.test.ts may fail an unrelated CI shard; ignore it.

^{Written for commit 5b662fe. Summary will update on new commits.}