Migrate from stdlib flag to cli.Main() + WithCommands() pattern:
- AddChainCommands() registration with persistent --data-dir/--seed/--testnet flags
- Explorer subcommand (TUI block explorer, replaces old default mode)
- Sync subcommand with headless foreground and --daemon/--stop modes
- sync_service.go extracts syncLoop/syncOnce from old main.go
- cmd/core-chain/main.go standalone binary entry point
- Add go-process dependency for daemon lifecycle
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
