Implement extended story hooks for mid-execution event injection

[lanycrost]

Summary

Extend the existing lifecycle hook mechanism (`steprun.ready`, `storyrun.ready`) with external hook injection via the transport hub.

Problem

Currently there is no way to inject events into a running workflow mid-execution. This blocks feedback loops, cross-workflow pub/sub, and plugin hooks without forking the operator.

Proposed change

• External hook injection points at well-defined lifecycle stages
• Transport hub integration for hook event delivery
• Plugin/extension API for registering custom hooks

This is a foundational feature that unblocks:

• Feedback loops / cycles (known gap)
• Pub/sub between workflows (addressable gap)
• Plugin/extension hooks on controller (addressable gap)

References

• Roadmap: https://bubustack.io/docs/community/roadmap (Near-term vision)
• Lifecycle hooks: https://bubustack.io/docs/streaming/lifecycle-hooks