Wire cluster CLI subcommands into main.rs

[jeremymanning]

Parent: #7

Wire cluster status CLI (status, peers, ledger-head). Currently prints 'not yet implemented'.

Files: src/main.rs, src/cli/ (needs cluster subcommand struct)

[jeremymanning]

Resolved by PR #58 (spec 003-stub-replacement). All 77 tasks complete, 489+ tests passing, CI green on Linux/macOS/Windows.