Isolate npm exec for self-hosted UAT

[thossullivan]

Root cause

The first public live-UAT run reached a GitHub runner but the plan job exited 127. Because the target checkout itself is model-eol@0.4.0, npm treated that project as satisfying --package=model-eol@0.4.0 without creating an executable bin.

This is specific to self-dogfood; the published command works from a neutral consumer directory.

Fix

Give npm exec an empty runner-temp prefix while leaving the checkout as the command working directory. Plan paths and independently regenerated bot plans therefore remain repo-relative and identical.

Validation

• exact workflow command resolves published model-eol@0.4.0
• plan has one item, zero issues, and zero scan notes
• isolated evaluator passes with its required report
• workflow YAML parses