feat: add bd admin parent command for cleanup/compact/reset (bd-3u8m)

Move cleanup, compact, and reset commands under `bd admin` namespace.
Creates hidden aliases for backwards compatibility that show deprecation
notice when used.

- Create cmd/bd/admin.go with parent command
- Create cmd/bd/admin_aliases.go for hidden backwards-compat aliases
- Update cleanup.go, compact.go, reset.go to remove rootCmd.AddCommand
- Update all documentation to use `bd admin <cmd>` syntax

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/reset.go

@@ -15,9 +15,8 @@ import (
 )
 
 var resetCmd = &cobra.Command{
-	Use:     "reset",
-	GroupID: "advanced",
-	Short:   "Remove all beads data and configuration",
+	Use:   "reset",
+	Short: "Remove all beads data and configuration",
 	Long: `Reset beads to an uninitialized state, removing all local data.
 
 This command removes:
@@ -37,7 +36,7 @@ Examples:
 
 func init() {
 	resetCmd.Flags().Bool("force", false, "Actually perform the reset (required)")
-	rootCmd.AddCommand(resetCmd)
+	// Note: resetCmd is added to adminCmd in admin.go
 }
 
 func runReset(cmd *cobra.Command, args []string) {