Files changed since the session's agent read them, reported again until it reads them anew so an ignored notice cannot get lost.
Whether uri differs from what the session's agent last saw. false if never read or gone: neither holds content a write could discard.
Snapshots uri as the session's agent just saw it, or forgets it if it is gone. Pass content when at hand to save a read.
Optionalcontent: string
Tracks the content an agent has seen per chat session, so that changes by anyone else can be reported back to it.