[codex] Split guard quality and platform tests

[PerishCode]

Why

The guard workflow currently runs fmt, clippy, tests, and flavor self-check on every OS in the matrix. Recent runs show Windows is consistently the slowest path, while fmt and clippy provide little extra signal when repeated across platforms.

What

• Replace the all-OS full guard matrix with a Ubuntu-only quality job.
• Keep fmt, clippy, workspace tests, and flavor self-check in the quality job.
• Add a Windows/macOS cross-platform test matrix that runs only cargo test.

Tests

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/guard.yml"); puts "yaml ok"'
• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo test --locked --workspace
• cargo run --locked -p flavor-cli -- check --root . --config flavor.json