Add loop primitive for bounded iteration with exit conditions

[lanycrost]

Summary

Implement bounded iteration as a new step primitive with exit conditions.

Problem

Users currently must chain `executeStory` with conditions for retry/iteration scenarios. There is no native loop construct.

Proposed change

Add a loop primitive that:

• Supports bounded iteration with configurable max iterations
• Evaluates exit conditions after each iteration
• Internally uses recursive `executeStory` so it does not violate the DAG model
• Integrates with the existing primitives support matrix

References

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