feat: pick the share key in Go to, or by reference

Sharing without share.key opens Go to as a picker of secrets instead
of asking for a link: it lists only secrets, enter picks one, and a
pasted reference picks it too. The key, there and in share.key or
--key, can be a secret's lazysecrets:// link, Key Vault URL or
@Microsoft.KeyVault(...) reference.

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

internal/ui/app/app.go

@@ -273,9 +273,7 @@ type Model struct {
 	linked     []provider.Container // vaults opened from links, this session
 	open       *ref.Ref             // reference to open at startup
 
-	// shareKey is the config's share.key; lastShareKey the key last
-	// typed when sharing, offered next time.
-	shareKey, lastShareKey string
+	shareKey string // the config's share.key
 
 	// A lazysecrets:// link found on the clipboard, offered in the footer.
 	dismissedLink string // the last link offered: not offered again