Replace env-heavy payload injection with mounted artifact bundles

[lanycrost]

Summary

Replace the current environment-variable-based payload injection with mounted runtime bundles backed by Secret, ConfigMap, or shared storage depending on sensitivity and size.

Problem

Current approach injects payloads via environment variables, which:

• Causes pod-template churn on config changes
• Makes large evaluated configs hard to inspect
• Complicates SDK/runtime loading

Proposed change

Mount runtime bundles as volumes instead of environment variables. The bundle format, mount path, and sensitivity classification (Secret vs ConfigMap vs storage) should be configurable per-step.

References

• Roadmap: https://bubustack.io/docs/community/roadmap (Near-term vision)
• Payloads docs: https://bubustack.io/docs/runtime/payloads