[codex] Improve service conflict suggestions

[Jonesxq]

Summary

• make existing-service conflict suggestions choose the rename option that matches the requested output path
• explain that --skip-services-check replaces the conflicting service file and include its path
• add regression tests for full-file and directory output modes

Why

The previous suggestion always recommended both --file and --name, even when only one option applied. It also described --skip-services-check as acceptable without explaining which existing service file would be replaced. This made conflicts confusing when the output path itself was not the source of the conflict.

Validation

• cargo fmt --verbose --check
• cargo test --verbose (79 tests passed)
• cargo clippy -- -Dwarnings on Linux
• git diff --check

Closes #212