test: make CLI write error assertion deterministic

[tbrandenburg]

Motivation

• Avoid flaky test behavior caused by relying on privileged filesystem paths like /root which vary by environment.
• Ensure the CLI write-error test triggers a deterministic failure mode that does not depend on permissions.
• Harden the test assertion so it reliably inspects CLI error output across different Node/spawn wrappers.

Description

• Updated src/cli/index.test.ts to create a local output-dir under the test tempDir and pass that directory to the CLI -o option to trigger an EISDIR write error deterministically.
• Replaced the previous privileged path usage with the deterministic directory write failure approach in the should handle file write errors gracefully test.
• Improved the error assertion to read stderr, stdout, or error.message and assert that the output contains the stable message Failed to write file:.

Testing

• Ran make qa, which executes lint, format, build, and vitest run as part of the QA pipeline.
• All automated checks completed successfully and vitest finished with all tests passing (full test suite passed).

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Codex Task