feat: picking the key shares, with no second popup

The picker already shows what's shared. The confirmation stays for a
key from share.key, which wasn't picked. The share format's version
is documented as kept readable.

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

internal/share/share.go

@@ -13,6 +13,10 @@
 //	version (1) | uvarint length of the header | header (JSON) | age file
 //
 // The age file holds the files as deflated JSON.
+//
+// A change to the format gets the next version, and Read keeps reading
+// every earlier one: links stay in chat and tickets for years. A
+// version it doesn't know is from a newer lazysecrets.
 package share
 
 import (
@@ -36,6 +40,7 @@ import (
 )
 
 const (
+	// version is the format's, written first; see the package doc.
 	version = 1
 	// MaxLink keeps a link short enough for chat and tickets (Teams takes
 	// about 28 KB) and for passing it to a terminal window.