feat: add --watch flag to stream live storage changes

Problem

Users monitoring on-chain state must repeatedly re-run queries manually. There's no way to subscribe to live storage changes from the CLI.

Related

• #69 (watch command)

Proposed Solution

Add a --watch flag to the query command that streams storage updates in real-time.

Implementation

polkadot-api provides watchValue() and watchEntries() Observables that emit on every storage change for the subscribed key.

if (options.watch) {
  const observable = storage.watchValue(...keys)
  observable.subscribe({
    next: (value) => printResult(value, options),
    error: (err) => { console.error(err); process.exit(1) },
  })
  // Keep process alive until SIGINT
} else {
  const value = await storage.getValue(...keys)
  printResult(value, options)
}

Behavior

• Each update emits a new result via printResult
• Graceful SIGINT (Ctrl+C) handling to cleanly unsubscribe and disconnect
• Pairs naturally with --output ndjson (see #64) — one JSON line per update
• Pretty output shows a timestamp separator between updates

Priority

MEDIUM

Files

• src/commands/query.ts

Start in src/commands/query.ts and trace the existing query path, including printResult and storage.getValue. Add the --watch path using the described storage observable, then verify that updates are emitted, SIGINT unsubscribes and disconnects cleanly, and ndjson output remains one line per update.