fix(release): finalize gmutils v0.6.0 metadata

[eareimu]

Summary

Follow-up release preparation for gmutils v0.6.0 after the first release PR merged.

This PR keeps the approved v0.6.0 contents and adds the final publish metadata required before the initial crates.io publication and final tag:

• complete crates.io metadata for all publishable gmutils crates;
• switch public package/license metadata to Apache-2.0;
• add root README and Apache-2.0 LICENSE text;
• add Debian copyright metadata;
• make S3 publish workflow command construction tag-mode safe for deb, rpm, scoop, and Homebrew;
• add xtask guard tests for workflow publish commands and package license metadata.

Release plan

• Target branch: main
• Final tag: v0.6.0
• Initial release surfaces: crates.io and S3 product packages
• crates.io first publication remains a post-merge, pre-tag manual/agent publication gate.

Local verification

• cargo +nightly fmt -- --check
• git diff --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-targets --all-features
• cargo publish --workspace --exclude xtask --dry-run --locked