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/fake/repos.go

@@ -134,3 +134,18 @@ func format(path string) string {
 	}
 	return "dotenv"
 }
+
+var _ provider.Contents = (*Provider)(nil)
+
+// Contents is a plain env file as KEY=value lines, from its keys, so the
+// demo repos' env files can be shared.
+func (p *Provider) Contents(ctx context.Context, c provider.Container) ([]byte, error) {
+	if c.Detail != provider.DetailPlainEnv {
+		return nil, provider.ErrUnsupported
+	}
+	var b strings.Builder
+	for _, o := range p.objects[listKey{c.ID, provider.TypeSecret}] {
+		b.WriteString(o.Name + "=" + p.values[o.ID] + "\n")
+	}
+	return []byte(b.String()), ctx.Err()
+}