feat: add Slack built-in MCP preset

[Ayush7614]

Summary

• Adds a Slack built-in MCP preset to src/mcp/presets.ts (closes Add a new built-in MCP preset (Slack, Linear, Sentry, or Notion) #3 for Slack).
• Uses npx -y @modelcontextprotocol/server-slack with SLACK_BOT_TOKEN + SLACK_TEAM_ID.
• README MCP preset list updated; unit test covers registration.

Test plan

• bun run typecheck
• bun test
• bun run build
• With valid Slack env vars: /mcp enable slack and confirm tools connect

[github-actions]

🤖 KlaatAI Review Bot (powered by Klaatu, advisory only — a maintainer makes the real call)

Issue match
The PR adds the Slack preset to src/mcp/presets.ts and updates README.md, satisfying the core requirement. However, it misses the issue's explicit acceptance criterion to update the docs site at content/docs/mcp.md. Partial match.

Test coverage
The new src/mcp/presets.test.ts verifies the Slack preset's registration, exact envVars (SLACK_BOT_TOKEN, SLACK_TEAM_ID), command arguments, case-insensitive lookup, and ID uniqueness. This thoroughly covers the structural correctness of the config addition. Runtime connection testing is appropriately omitted for a config-only change.

Correctness concerns
In src/mcp/presets.ts, the envVars array includes SLACK_TEAM_ID in addition to SLACK_BOT_TOKEN. The linked issue's suggested template only listed SLACK_BOT_TOKEN. Reviewers should verify if SLACK_TEAM_ID is strictly required by the upstream @modelcontextprotocol/server-slack package. If it is optional or incorrect, enforcing it in the preset definition could break the setup for users who don't set it.

Verdict
Needs a specific fix first: add the missing content/docs/mcp.md update to satisfy the issue's acceptance criteria, and confirm whether SLACK_TEAM_ID is actually required by the upstream MCP server.

_{This is an automated review to help triage faster, not a gate. Nothing here blocks merging.}