feat(apiproxy): workspace.archiveSession RPC and archived-sessions frame

workspace.archiveSession answers the full updated archive set;
workspace.list carries the set as the reconnect baseline; the host
stream pushes host/archived-sessions-changed full snapshots from the
domain/changed global-put branch (same posture as workspace-changed).
Unknown sessions map to the existing session-not-found code.

packages/host/apiproxy/src/api/events.ts

@@ -101,7 +101,9 @@ export type MuxFrame =
  * workspace mutation (create/attach/order change — the client upserts, while
  * `workspace.list` provides the reconnect baseline); workspace-removed is the
  * committed registration-deletion increment and never implies directory or
- * session-log deletion.
+ * session-log deletion; archived-sessions-changed pushes the full registry
+ * archive set after every durable change (same full-snapshot posture as
+ * workspace-changed — `workspace.list` re-baselines it on reconnect).
  */
 export type HostFrame =
   | { type: 'host/session-added'; sessionId: SessionId; blank: boolean; parentSessionId?: SessionId; cwd?: string }
@@ -110,6 +112,7 @@ export type HostFrame =
   | { type: 'host/agent-error'; sessionId: SessionId; message: string }
   | { type: 'host/workspace-changed'; workspace: WorkspaceView }
   | { type: 'host/workspace-removed'; workspaceId: WorkspaceView['workspaceId'] }
+  | { type: 'host/archived-sessions-changed'; archivedSessionIds: SessionId[] }
   /**
    * The command registry changed (`commands/change` passthrough). Pure
    * invalidation signal, no payload: clients refetch `command.list` in the