fix: surface deploy command failures in action output

[whyujjwal]

Summary

• surface the original zx/deployer process failure message instead of replacing it with a generic Failed: dep ... error
• export main behind an entrypoint guard so the module can be tested without auto-running
• add a regression test proving the original deployer failure text is preserved

Fixes #21

Validation

• npm test
• npm run typecheck
• npm run format:check
• npm run build