"My" Cosigners vs Others

Unclear on exact naming and UX, but clearly some Cosigners without `xprv`s (or a seed) are "mine", and thus are a priority for a `crypto-request` after a PSBT is imported and the user clicks the Sign button, or when other apps `crypto-request` to sign a PSBT.

An open question is if Cosigners with `xprv`s are always "mine". Certainly they should be `mine` by default when imported, but maybe you'll want to turn certain ones off.

I'm also pretty sure that "my" or "mine" is the wrong naming. Maybe @shannona has some inspiration.

The issue describes unresolved naming and UX for identifying which Cosigners are “mine,” including imported PSBT signing and incoming crypto-request flows. No files, tests, or entry points are named; first resolve the terminology and ownership rules, then define completion criteria for the affected user flows.

Written by the indexing model from the issue text.