Add pipe-friendly output / export helpers

[swinton]

There's no way to get data out of the REPL into other tools today. Export helpers would enable inbox analysis in spreadsheets, databases, or CLI pipelines.

Proposed API

// Export messages to JSON (stdout or file)
await toJSON(await messages('from:boss@example.com'))

// Export messages to CSV
await toCSV(await messages('from:boss@example.com'))

// Optionally write to a file
await toCSV(await messages('label:receipts'), 'receipts.csv')

Notes

• toJSON could just be a thin wrapper around JSON.stringify with pretty-printing
• toCSV should flatten the key fields: id, from, to, subject, date — pairs naturally with decode() added in Add send, reply, and draft capability #2
• Writing to stdout makes it pipeable: ./repl.js < script.js > output.csv
• No new dependencies needed for JSON; CSV can be hand-rolled for the simple flat structure we need