Refactor internal module structure

[ebihara99999]

Summary

• Move runtime diagnostic redaction and action report output into a dedicated diagnostics module.
• Extract shared path helpers and stow action/report types while preserving the existing rustow::stow::* public paths.
• Simplify the binary entrypoint and split config construction into smaller helper functions.

Validation

• cargo fmt --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo test --locked
• cargo build --release --locked
• cargo test --release --locked
• cargo deny check bans licenses sources