Actions Runner Controller 0.15.0 [GA]

### Value Prop
Actions Runner Controller (ARC) 0.15.0 makes it easier to configure, operate, and troubleshoot self-hosted runners on Kubernetes — with cleaner Helm chart configuration, improved logging, and a batch of fixes and improvements requested directly by the ARC community. Platform teams spend less time wrestling with rough edges and more time shipping. This release incorporates vetted community contributions, so the improvements reflect real-world usage at scale.

### Expected Outcome
Teams running GitHub Actions on Kubernetes gain a more stable and polished ARC experience, with fewer day-to-day friction points and better visibility when things go wrong. By consistently addressing community-reported issues and merging community pull requests, this release builds confidence in the ARC release cadence and migration path away from legacy ARC.

This roadmap issue covers the Actions Runner Controller 0.15.0 release for GitHub Actions on Kubernetes. Start by identifying the release scope, including Helm chart configuration, logging, fixes, and community contributions; the payload names no files, tests, or entry points. Done would require delivering the stated release improvements and validating the expected stability and troubleshooting outcomes.

索引モデルが issue の本文から書いたものです。