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/admin.go

@@ -0,0 +1,26 @@
+package main
+
+import (
+	"github.com/spf13/cobra"
+)
+
+var adminCmd = &cobra.Command{
+	Use:     "admin",
+	GroupID: "advanced",
+	Short:   "Administrative commands for database maintenance",
+	Long: `Administrative commands for beads database maintenance.
+
+These commands are for advanced users and should be used carefully:
+  cleanup   Delete closed issues and prune expired tombstones
+  compact   Compact old closed issues to save space
+  reset     Remove all beads data and configuration
+
+For routine operations, prefer 'bd doctor --fix'.`,
+}
+
+func init() {
+	rootCmd.AddCommand(adminCmd)
+	adminCmd.AddCommand(cleanupCmd)
+	adminCmd.AddCommand(compactCmd)
+	adminCmd.AddCommand(resetCmd)
+}