fix(nexus): return portable image artifact paths

[smackgg]

Summary

• Return Nexus image artifacts as portable paths relative to the host-provided output directory.
• Keep native absolute paths private to the companion writer.
• Add a regression test for the artifact-path contract.
• Release Nexus plugin v0.3.7 with companion v0.3.6.

Root cause

The provider response was accepted and the image bytes were written, but the companion returned an absolute artifact path. Convax correctly rejected it during output admission, after the external request had completed, so the operation became retry-safety unknown to avoid duplicate billing.

Verification

• bun run check
• Nexus companion: 20 tests / 74 assertions
• Regression test asserts a relative artifact path and reads the generated file through output_directory.