docs: enhance package comment in internal/beads/beads.go (gt-bi7nb)

Add usage guidance to package-level comment explaining the primary entry
point (New()) and key methods for managing beads issues.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: nux

internal/beads/beads.go

@@ -1,4 +1,6 @@
 // Package beads provides a wrapper for the bd (beads) CLI.
+// Use New() to create a Beads instance for a working directory, then call
+// methods like List(), Show(), Create(), Update(), and Close() to manage issues.
 package beads
 
 import (