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>

docs/FAQ.md

@@ -333,10 +333,10 @@ Use compaction to remove old closed issues:
 
 ```bash
 # Preview what would be compacted
-bd compact --dry-run --all
+bd admin compact --dry-run --all
 
 # Compact issues closed more than 90 days ago
-bd compact --days 90
+bd admin compact --days 90
 ```
 
 Or split your project into multiple databases: