[Feature] Action buttons — suggested run: commands as clickable Slack buttons

[davesleal]

When the agent suggests a follow-up action like @Shellack run: summarize the project, it should render as a clickable Block Kit button, not just a code block.

Clicking the button would auto-send that command as a thread reply, triggering the run: session.

This builds on the existing claude_bridge_input action handler pattern.