Render Block Kit messages from apps and bots

[m96-chan]

Description

Messages from Slack apps and bots often use Block Kit for rich layouts (sections, buttons, dividers, context blocks, etc.). These are currently not rendered and may show as empty or raw text.

Proposed Solution

Implement a best-effort TUI renderer for common Block Kit elements:

• section — text with optional accessory
• divider — horizontal rule
• context — small text/images
• header — bold header text
• rich_text — formatted text blocks
• actions — display button labels (non-interactive in TUI)
• image — show alt text or URL

Interactive elements (buttons, menus) can display labels as non-clickable text since TUI cannot handle Slack interactivity.

Impact

Many workspaces rely heavily on bot messages (CI/CD, alerts, workflows). Without Block Kit rendering, these messages are unreadable.