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/QUICKSTART.md

@@ -178,16 +178,16 @@ As your project accumulates closed issues, the database grows. Manage size with
 
 ```bash
 # View compaction statistics
-bd compact --stats
+bd admin compact --stats
 
 # Preview compaction candidates (30+ days closed)
-bd compact --analyze --json --no-daemon
+bd admin compact --analyze --json --no-daemon
 
 # Apply agent-generated summary
-bd compact --apply --id bd-42 --summary summary.txt --no-daemon
+bd admin compact --apply --id bd-42 --summary summary.txt --no-daemon
 
 # Immediately delete closed issues (CAUTION: permanent!)
-bd cleanup --force
+bd admin cleanup --force
 ```
 
 **When to compact:**