v0.3.0 — hot registration + archiving

First public release.

• register: place an externally started Session under a Workspace and publish its summary — the row appears without restarting the app.
• archive / unarchive: hide or restore a Session through the Host registry. Files are never deleted; unarchive restores the original position.
• File-inbox protocol (requests/*.json → receipts/*.json): no ports, no auth surface, usable from any language.
• The Host stays the only writer of workspace.json; the plugin only calls the public registry API.