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/CONFIG.md

@@ -576,7 +576,7 @@ jira_project = get_config("jira.project")
 
 Some bd commands automatically use configuration:
 
-- `bd compact` uses `compact_tier1_days`, `compact_tier1_dep_levels`, etc.
+- `bd admin compact` uses `compact_tier1_days`, `compact_tier1_dep_levels`, etc.
 - `bd init` sets `issue_prefix`
 
 External integration scripts can read configuration to sync with Jira, Linear, GitHub, etc.