## Why
`thread/attachment/list` lists attachments for a known thread. Clients also need to find the threads associated with a given attachment identity.
## What changed
Add `thread/attachmentOwner/list` to return owning thread IDs and archive status for an exact `attachmentType` and `identityKey`. Support cursor pagination and an optional `archived` filter, including both archived and non-archived threads by default.
Wire the request through the thread store and update documentation, protocol schemas, and generated TypeScript and Python types. The lookup is limited to the configured thread store and reports current membership, not an atomic resource-cleanup check.
## Testing
Extend app-server integration tests to cover exact type-and-key matching, multiple owners, archive filtering, and invalid cursor rejection.
GitOrigin-RevId: 30f4a20e00243d0023246d465a9b95213b3adeb6
