Add agent thread sharing (#46140)

Staff only ship for now

Here's the agent planning doc that guided this:
https://gist.github.com/mikayla-maki/c826b7997bd85b58273c1def9397940b

Release Notes:

- N/A

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Co-authored-by: Marshall Bowers <git@maxdeviant.com>
3da926981cMikayla Maki committed on 1/6/2026, 8:49:51 PM· committed by GitHubparent8df2789
30 files changedLine totals unavailable