Adds `message draft` CLI command that opens a WYSIWYG editor in the
browser for composing Slack messages with full rich text support (bold,
italic, strikethrough, code, code blocks, lists, blockquotes, links).
Features:
- Self-contained HTML editor served via ephemeral HTTP server
- Slack dark theme with Agent Slack branding
- Toolbar + keyboard shortcuts matching Slack (Cmd+B/I/E/K/etc.)
- Code block escape via double-Enter
- Inline link popover with Text + URL fields (Cmd+K)
- mrkdwn source mode toggle
- Thread context with Slack hyperlink
- Sends message directly via Slack API on submit
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Neil