feat(editor): extract chat runtime (#14937)

#### PR Dependency Tree


* **PR #14937** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

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## Summary by CodeRabbit

* **New Features**
* Centralized AI event system and a runtime powering chat sessions and
actions.

* **Improvements**
* Chat UI (composer, messages, toolbar, tabs, panels) now syncs with
runtime snapshots for more consistent state.
* Improved session/tab lifecycle (create, fork, delete), context
embedding status, and history handling.
* More reliable send/stop/retry flows, better telemetry scoping, and
clearer upgrade/login/insert-template prompts.
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