[kvinwang]

Summary

Use the repository's existing safe-write dependency for attestation output files instead of maintaining another atomic-write implementation.

Changes

• Write quote reports, encoded attestations, JSON attestations, and stripped attestations with safe_write::safe_write.
• Support text and binary output through the library's existing byte-oriented API.
• Remove the extra dstack-cli-core dependency and custom byte-write helpers from this PR.

Verification

• cargo check --manifest-path dstack/Cargo.toml -p dstack-util
• cargo fmt --manifest-path dstack/Cargo.toml --all
• git diff --check

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