Feature: Add `unfurl_links` and `unfurl_media` parameters to `slack_send_message`

[kaarelr]

Problem

slack_send_message doesn't expose Slack's unfurl_links / unfurl_media parameters from chat.postMessage. Every URL in a message generates a large preview card. When an AI agent posts a message with several links (e.g. Linear tickets, Slack archive links, GitHub PRs), the unfurled previews create a wall of cards that buries the actual message content.

There's no mrkdwn-level workaround — unfurl_links: false on the API call is the only way to suppress previews while keeping links clickable.

Proposed solution

Add two optional boolean parameters to the slack_send_message tool:

• unfurl_links (default: true) — controls unfurling of text-based content
• unfurl_media (default: true) — controls unfurling of media content

These map directly to the existing chat.postMessage parameters.

Ideally also add them to slack_send_message_draft and slack_schedule_message.

Use case

AI agents (Cursor, Claude Code) posting investigation summaries or triage responses in Slack threads. These messages routinely reference 3-5 URLs. The unfurled previews take up more screen space than the message itself.

[cchensh]

cc @stevengill , @manucaicedo , this seems a good feature request. It's a straightforward pass-through of existing chat.postMessage parameters, low risk, and solves a real UX problem for agent-heavy workflows where messages are link-dense.