Make prompt library icon in context panel staff-only for now (#12457)

This is still pretty raw, so I'd like to hold off on shipping it to all
users.

Release Notes:

- Hide the prompt library for non-staff until it is in a more complete
state.

crates/assistant/Cargo.toml

@@ -22,6 +22,7 @@ client.workspace = true
 collections.workspace = true
 command_palette_hooks.workspace = true
 editor.workspace = true
+feature_flags.workspace = true
 file_icons.workspace = true
 fs.workspace = true
 futures.workspace = true