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>

commands/compact.md

@@ -12,11 +12,11 @@ Reduce database size by summarizing closed issues no longer actively referenced.
 
 ## Usage
 
-- **Preview candidates**: `bd compact --dry-run`
-- **Compact all eligible**: `bd compact --all`
-- **Compact specific issue**: `bd compact --id bd-42`
-- **Force compact**: `bd compact --id bd-42 --force` (bypass age checks)
-- **View statistics**: `bd compact --stats`
+- **Preview candidates**: `bd admin compact --dry-run`
+- **Compact all eligible**: `bd admin compact --all`
+- **Compact specific issue**: `bd admin compact --id bd-42`
+- **Force compact**: `bd admin compact --id bd-42 --force` (bypass age checks)
+- **View statistics**: `bd admin compact --stats`
 
 ## Options