feat: add `dot watch` command for real-time block, storage, and event streaming

Parent Epic: #63

Summary

Add a watch command that streams real-time chain data as NDJSON, enabling agents to monitor blocks, storage changes, and events.

Usage

dot watch blocks --output json
dot watch storage System.Account 5FHn... --output json
dot watch events --output json
dot watch events Balances.Transfer --output json

Output (NDJSON)

Blocks:

{"blockNumber":12345,"blockHash":"0x...","parentHash":"0x...","extrinsicsCount":5}
{"blockNumber":12346,"blockHash":"0x...","parentHash":"0x...","extrinsicsCount":3}

Storage:

{"blockNumber":12345,"value":{"nonce":42,"data":{"free":"1000000000000"}}}
{"blockNumber":12350,"value":{"nonce":43,"data":{"free":"999000000000"}}}

Events:

{"blockNumber":12345,"pallet":"Balances","event":"Transfer","fields":{"from":"...","to":"...","amount":"1000"}}

Implementation

• Use polkadot-api's client.finalizedBlock$ for block streaming (same RxJS pattern as watchTransaction in tx.ts)
• Use polkadot-api's watchValue for storage subscriptions
• Emit NDJSON lines using printJsonLine() from output.ts
• Support Ctrl+C graceful shutdown

Files

• src/commands/watch.ts — NEW command with blocks, storage, events subcommands
• src/cli.ts — register new command

Verification

• Stream blocks for 30 seconds, verify each line is valid JSON
• Watch a storage value, trigger a change via tx, verify the update appears
• Filter events by pallet, verify only matching events are emitted

Start in src/commands/tx.ts at the watchTransaction RxJS pattern, then inspect src/output.ts for printJsonLine() and src/cli.ts for command registration. Implement the blocks, storage, and events subcommands in src/commands/watch.ts using finalizedBlock$ and watchValue, and verify valid NDJSON, storage updates, event filtering, and graceful Ctrl+C shutdown.