Add bd cleanup command for bulk deletion of closed issues

Implements bd-jx90: simple cleanup command without API key requirement.

Features:
- Delete all closed issues with --force
- Filter by age with --older-than N (days)
- Preview with --dry-run
- Cascade deletion with --cascade
- JSON output support

Usage:
  bd cleanup --force                    # Delete all closed
  bd cleanup --older-than 30 --force    # Delete >30 days old
  bd cleanup --dry-run                  # Preview

Closes bd-jx90

Amp-Thread-ID: https://ampcode.com/threads/T-8d905db0-4ec7-411e-95de-1f044219dc9c
Co-authored-by: Amp <amp@ampcode.com>

AGENTS.md

@@ -245,6 +245,12 @@ bd close <id> [<id>...] --reason "Done" --json
 # Reopen closed issues (supports multiple IDs)
 bd reopen <id> [<id>...] --reason "Reopening" --json
 
+# Clean up closed issues (bulk deletion)
+bd cleanup --force --json                                   # Delete ALL closed issues
+bd cleanup --older-than 30 --force --json                   # Delete closed >30 days ago
+bd cleanup --dry-run --json                                 # Preview what would be deleted
+bd cleanup --older-than 90 --cascade --force --json         # Delete old + dependents
+
 # Show dependency tree
 bd dep tree <id>