v0.8.6 — stand-in signatures stay behind the fold

Fixed

• Gemini/Messages folds: a thinking block that only carries the fold's stand-in signature — the marker the response fold writes onto a rebuilt thought so Messages clients keep it as thinking across a replay — is now folded as an unsigned block (a bare thought part) instead of being sent to the target as a thoughtSignature. The target validates what it is handed and answers INVALID_ARGUMENT, which a gateway surfaces as a rejected model request: a Messages client behind a gateway lost the second request of every multi-turn tool loop once a rebuilt thought was in history.
• The same stand-in on a replayed tool_use call folds to the target's skip marker, the value a missing signature already gets.
• The streamed request fold drops a stand-in signature delta instead of emitting it as a thoughtSignature part.

A client that never replays a block is unaffected: real signatures still ride through untouched, in either spelling.

Full history: CHANGELOG.md.