feat: share env files in --demo

Providers whose containers are files give their contents through
provider.Contents, which sharing reads instead of the disk. The demo
repos build their env files from their keys, and the second demo
account has a key to share with (kv-shared-prod / env-share), so S → f
and opening the link work in --demo; the demo repos aren't on disk,
so opening asks where to save.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/provider/provider.go

@@ -189,6 +189,12 @@ type Files interface {
 	File(c Container, o *Object) (File, error)
 }
 
+// Contents is implemented by providers whose containers are files, so
+// they can be shared (package share): the file as it is.
+type Contents interface {
+	Contents(ctx context.Context, c Container) ([]byte, error)
+}
+
 // File is a container's file, to open.
 type File struct {
 	Path string