Remote Control placeholder clean-up checks the server before archiving

What

Remote Control lets you drive a Claude Code session from another device. It can create placeholder sessions, and a clean-up pass (the sweep) later removes or archives the ones left over. That pass is now stricter:

• Placeholder records now store optional titleSha (a fingerprint of the title) and usedAt (when it was used) fields.
• A placeholder whose session is connected, or whose state cannot be read, is kept.
• A placeholder is removed when it was used, retitled, or its updated_at time changed. Sessions that are active but disconnected are also removed.
• The local record is read again just before archiving.
• The pass reports how many sessions were not archived because of their server state, and a stamping failure shows as "[bridge:placeholder] use stamp failed".

Before, a placeholder was removed when updated_at differed from created_at and otherwise archived, using timestamps only.

Why

This stops the clean-up from archiving a Remote Control session that is still connected or has already been used.