ci: route workflows through Nexus runners

[neuralnexus]

Summary

• allow same-repository pull requests to use NEXUS_RUNS_ON_LINUX when configured
• keep fork pull requests on hosted ubuntu-26.04
• preserve the existing single-job workflow semantics

Validation

• actionlint
• YAML parse
• git diff --check

[neuralnexus]

Runner routing correction verified.

The repository Actions runners API is accessible but returned total_count: 0; NEXUS_RUNS_ON_LINUX therefore remains unset. The PR branch has no direct generic Linux arrays, so no follow-up commit was needed.