fix: restrict downloaded file permissions

[jbeckwith-oai]

Summary

• Create newly downloaded files with owner-only permissions for explicit output paths and server-provided filenames.
• Preserve existing output-file permissions and overwrite behavior, filename collision handling, temporary-file fallback, and traversal protections.
• Cover existing-file modes, permissive and restrictive umasks, encoded traversal, symlink collisions, and concurrent downloads with synthetic fixtures.

Verification

• Focused download tests under umasks 000, 022, and 077.
• go test -race ./pkg/cmd -run '^(TestWriteBinaryResponse|TestCreateDownloadFile)$' -count=5
• All six generated download endpoint integration tests.
• ./scripts/lint, go vet ./..., and formatting checks.
• Full repository and race suites, excluding one pre-existing macOS FIFO cancellation failure independently reproduced on the unchanged base commit.
• Windows amd64/arm64 and Linux amd64/arm64 compilation.

[Copilot]

Pull request overview

This PR tightens default filesystem permissions for downloaded output artifacts created by the CLI, ensuring newly created files are owner-only readable/writable while preserving existing overwrite behavior and filename-collision handling.

Changes:

• Create explicit output files (when --output is a path) with 0600 permissions instead of 0644.
• Create server-suggested download filenames (via Content-Disposition) with 0600 permissions when the file is newly created.
• Expand download-related tests to assert permissions, validate overwrite-mode preservation, and cover traversal/symlink/collision/concurrency scenarios.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pkg/cmd/cmdutil.go	Switch new output/download file creation modes from 0644 to 0600 for tighter default permissions.
pkg/cmd/cmdutil_test.go	Add/extend tests to verify permission behavior and robustness across overwrite, traversal, symlink, and concurrent download cases.

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[HAYDEN-OAI]

Independent security review of the exact requested commit: no introduced P1/P2 findings. Verified owner-only file creation, preserved overwrite behavior, collision and traversal protections, stdout, failure paths, and concurrent downloads with direct base-versus-HEAD reproductions.