feat: add download subcommand for Slack files (0.4.0)

[nori-sessions]

Summary

🤖 Generated with Nori

• Adds a first-class nori-slack download --id <F123> --output <path> subcommand that resolves a file and writes its raw bytes to disk, replacing the old base64 files.download proxy hack. (Uses --id, deliberately not --file, to avoid colliding with upload --file.)
• Transport gains downloadFile() with proxy- and direct-mode implementations. Proxy mode hits the broker's new GET /slack-proxy/download (the session has no bot token); direct mode resolves files.info then fetches url_private_download with the bot token.
• Direct mode guards against Slack's HTTP-200 HTML sign-in page, which would otherwise be written to disk as a "successful" download when the token is rejected.
• Serializes test runs (--no-file-parallelism) to stop subprocess-based CLI tests flaking under vitest concurrency.

Note: this should be published to npm before the companion broker change (tilework-tech/nori-sessions#1140) deploys, since the broker's rewritten attached-files prompt instructs sessions to run nori-slack download.

Test Plan

• npm test (proxy-mode happy path + 403/404/401 error mapping + arg validation; direct-mode byte fetch + HTML-login-page rejection)
• npx tsc --noEmit clean
• Manual: nori-slack download --id F123 --output ./out.bin against a real broker proxy and a direct bot token