feat: schedule messages with browser auth

[dwaxe]

Summary

• keep standard tokens on the public chat.scheduleMessage API
• schedule browser sessions through Slack-native drafts.create with date_scheduled
• list and cancel native schedules through drafts.list and drafts.delete
• verify Enterprise Grid organization and user identity before routing native draft calls
• surface native-draft truncation and reject payloads Slack Desktop cannot preserve

Context

agent-slack already has the two required pieces from #102 and #119, but browser-session tokens cannot use chat.scheduleMessage. This connects the auth-aware paths while preserving Q... IDs for standard tokens and returning Dr... IDs for browser auth.

The native-draft behavior follows prior art from:

• https://github.com/tammersaleh/slack-cli/blob/ab04c76de1bd4eb00509a9d3c156ec8906db7588/docs/draft-messages.md
• https://github.com/emacs-slack/emacs-slack/blob/a7c971ef6f92ec1b9dca164512cd276a95cdfaa5/slack-scheduled-messages-buffer.el

Browser traffic continues to use the shared Slack API client; #142 remains independent transport hardening.

Native scheduled drafts accept only non-empty top-level rich_text blocks, reject --no-unfurl, and report has_more when the non-pageable 100-draft response may be incomplete.

Validated with 410 tests, typecheck, format, lint, and the Node-targeted build.