feat(api): add minimal Slack bot support

[devin-ai-integration]

feat(api): add minimal Slack bot support

Summary

Adds minimal Slack bot support to apps/api using @slack/bolt with Socket Mode. This is a skeleton setup that initializes the Slack app and starts it alongside the Hono server.

Changes:

• Added @slack/bolt dependency
• Created slack.ts with basic App initialization using Socket Mode
• Added required env vars: SLACK_BOT_TOKEN, SLACK_SIGNING_SECRET, SLACK_APP_TOKEN
• Updated .env.sample with new variables

Review & Testing Checklist for Human

• Breaking change: The new env vars are required (z.string().min(1)). Existing deployments will fail to start without these credentials. Consider making them optional or adding conditional initialization.
• No error handling: startSlackBot() is called without try/catch. If Slack credentials are invalid, the server may crash on startup.
• No bot functionality: This is just the connection skeleton with no listeners. You may want to add at least one command/event handler to verify it works.
• Test with valid Slack credentials to confirm the bot connects successfully

Notes

• The bot uses Socket Mode which doesn't require a public URL for webhooks
• To test: Create a Slack app at api.slack.com, enable Socket Mode, and add the three tokens to your .env

Link to Devin run: https://app.devin.ai/sessions/af8cf1a824144ffb88341acb769940c4
Requested by: yujonglee (yujonglee.dev@gmail.com) / @yujonglee

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch devin/1764042413-slackbot-support

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