feat(messages): support native Block Kit tables and Markdown (#124)

## Summary

- add `messages send --blocks <json|@file>` for inline or file-based
Block Kit arrays
- validate the top-level array and block `type` fields before sending
- pass native `table`, `markdown`, and other Block Kit blocks unchanged
to `chat.postMessage`
- JSON-encode structured request values for browser-session
authentication
- preserve existing text, thread reply, DM, and file-upload behavior
- document copyable native table and native Markdown examples

## Tests

- `bun test` — 421 passed, 0 failed
- `bun run type-check`
- `bun run build`
- `pre-commit run --all-files`

Closes #121