Separate experiment fixtures from runtime outputs

[jwalin-shah]

Summary

• Add a shared runtime output path helper that sends default support experiment results to ignored .runtime/experiments paths.
• Keep explicit output paths available for intentional fixture refreshes.
• Document fixture/runtime separation and add a regression test for support experiment defaults.

Validation

• python3 -m pytest -q
• git diff --check

Work pack

WP-172: Separate fixtures from runtime outputs

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.
To continue using code reviews, you can upgrade your account or add credits to your account and enable them for code reviews in your settings.