ci: use npm ci for app dependency installs

[ProfRandom92]

Motivation

• Make CI Node dependency installs reproducible by switching from a root-level npm install to app-scoped npm ci invocations for the dashboard and showcase apps.
• Keep the change narrowly scoped to workflow install steps to avoid altering build/test semantics or package artifacts.

Description

• Replaced root-level npm install with npm --prefix dashboard/app ci and npm --prefix showcase/app ci in the three CI workflows: .github/workflows/ci.yml, .github/workflows/agent-checks.yml, and .github/workflows/validation_runner.yml.
• Preserved existing Python install commands and retained npm run check, npm run typecheck, npm run build, and dashboard smoke-test steps unchanged.
• Only the three listed workflow files were modified and committed.

Testing

• Ran python scripts/run_checks.py, which completed successfully.
• Confirmed via git that only the three targeted workflow files changed and were committed.

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Codex Task

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.