feat: the key picker says what sharing does, and skips plain env values

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

internal/ui/app/modal.go

@@ -40,6 +40,7 @@ type option struct {
 	group  string        // the source row it's in (search)
 	kind   provider.Kind // the kind of provider it's from (search)
 	hits   []int         // characters of the label the filter matched (Go to)
+	plain  bool          // a key in a plain env file: no key to encrypt with (Go to)
 	run    func(m *Model) tea.Cmd
 	// header makes this a section title: not selectable, and hidden
 	// while a filter narrows the list.