ci: route workflows through Nexus runners

[neuralnexus]

Summary

• Route the PR-only Gemini job through vars.NEXUS_RUNS_ON_LINUX for same-repository pull requests.
• Keep fork pull requests on hosted ubuntu-26.04.
• Preserve the existing single job.

Configuration

• Leave NEXUS_RUNS_ON_LINUX unset because no matching online Linux runner is visible.

Validation

• Ruby YAML parse
• actionlint 1.7.12
• git diff --check
• Baseline comparison: job IDs and matrix row counts preserved

[neuralnexus]

Implementation status:

• Commit af7c32f8 is based directly on current main; only .github/workflows/gemini-review.yml changed.
• YAML parsing, actionlint 1.7.12, git diff --check, and the one-job comparison pass.
• Same-repository pull requests may use the selector; forks resolve to hosted ubuntu-26.04.
• Runner assignment recheck returned no visible assigned runners; NEXUS_RUNS_ON_LINUX remains unset and the round-trip variable listing is empty.
• Workflow branch audit found no embedded generic Linux selector, so no follow-up commit was needed.
• GitHub had not created an Actions run for this branch/PR at the time of verification.