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>

examples/compaction/README.md

@@ -171,7 +171,7 @@ Track compaction costs:
 
 ```bash
 # Show stats after compaction
-bd compact --stats
+bd admin compact --stats
 
 # Estimate monthly cost
 # (issues_compacted / 1000) * $1.00