Two instances working the same file, neither aware of the other, is a merge conflict you find out about an hour later. Presence is the cheap fix: instances say what they're touching, and the workspace shows it.
Instances report what they're on
A connected instance reports the areas it's working in - free-form strings, so whatever is meaningful: file paths, modules, topics, ticket refs. It names itself as it does so, which is what makes the display useful.
"Tell Kythene you're working on the billing module and the invoice templates."
The Claude skill teaches an instance to do this at the start of a piece of work without being asked.
Active now
The workspace shows recent instance activity - who and what, within a 30-minute window. Anything older has aged out, so it reads as "right now" rather than "ever".
Where two different producers have touched the same area inside that window, it's surfaced as a collision hint. Not a lock and not a block - just the information, early enough to be worth having:
"Anyone else working in the billing module right now?"
Catching that before you start is worth a lot more than catching it at merge.
Who read this
Reads of shared work are recorded too, which gives you lineage in the other direction: for a given collection or artifact, which instances have read it.
"Has anyone's instance actually read the migration plan?"
Two uses. Practically, it tells you whether the thing you published landed. For governance, it's part of answering "which shared knowledge did this output rely on?" with a record rather than a guess - see Provenance.
Privacy
Presence and read-logging are workspace-scoped and follow the same visibility rules as everything else: you only see activity on work you could see anyway. Reporting is a positive act by an instance - it reports the areas it names, not everything on your disk.