Add RecallObservations for retrieval of prompt extensions

[enyst]

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Cc: @xingyaoww
I'd love your opinion about this idea. It's basically an alternative to 6526, I was curious to see roughly how it looks like, so we can maybe see if it makes sense.

This PR proposes to refactor prompt extensions into recalled observations:

• take the retrieval of information out of the PromptManager, into a Memory component
• PromptManager remains responsible with loading templates and rendering them (just a 'view manager')
• Memory subscribes to the stream
  • on user messages, it may retrieve extensions and add them to the stream as RecallObservations
  • on the first user message, it may retrieve repo and runtime info
  • on recall actions (not yet in use, source=agent), it may retrieve e.g. library docs
• some logic is separated from the agent (like 6526)
• since the information is in the stream, session restore / refresh / runtime reconnect / etc will not lose it.

Not ready for review, just some food for discussion.

Cc: @csmith49

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Link of any specific issues this addresses
Fix #6535

[csmith49]

This looks clean so far. I'm a fan of moving what we can to the event stream: much better visibility than modifying messages in-place, and I think leaning into the pub-sub approach is a good way to add extra functionality without having to tip-toe around the agent/controller/system control flow.

[xingyaoww]

I like this idea better than what I did in #6526! Happy to close that PR in favor of this one