Closes AI-286

This PR adds a right-click menu to thread items in the sidebar. A big
motivation for this addition was surfacing the "regenerate thread title"
action. And because this action and the "open thread as markdown" action
previously only worked on the already-loaded agent panel, there was a
load of plumbing needed to make that work even for threads in workspaces
that haven't been loaded, mostly using the thread metadata store. At the
end of the day, the title regeneration feature is only possible for the
Zed agent, so this is gated from external agents. But I believe it's now
in a slightly nicer spot due to some refactors to better expose the
state in which the title regeneration can fall into.

Release Notes:

- Agent: Added a right-click menu to thread items in the sidebar,
allowing to regenerate a thread title, rename a thread title, open the
thread as markdown, and archive it.

---------

Co-authored-by: zed-zippy[bot] <234243425+zed-zippy[bot]@users.noreply.github.com>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>