Fix configured run target precedence

[giri256]

Fixes #3314

Summary

• Remove the yargs default from the array-valued run.target option so it is not mistaken for an explicit CLI value during config merging.
• Keep Firefox Desktop as the effective default in the run command when no target is provided.
• Add regression coverage for config precedence, explicit CLI precedence, and the undefined-target fallback.

Validation

• npx prettier --check src/program.js tests/unit/test.program.js tests/unit/test-cmd/test.run.js
• npx eslint src/program.js tests/unit/test.program.js tests/unit/test-cmd/test.run.js
• npx mocha tests/unit/test.program.js tests/unit/test-cmd/test.run.js (83 passing)

[giri256]

CI audit note: the only failing context is build-nodejs-current, at its final npm run audit-deps step. Prettier, unit/coverage tests, both functional-test modes, Node next/experimental, and Windows all pass.

The audit is reporting newly blocking advisories in existing dependencies (adm-zip, brace-expansion, fast-uri, and shell-quote); this PR does not modify dependency files. The same current-job failure pattern is also present on the repository's open Dependabot PRs #3780 and #3779, so this is not introduced by this change.