[OPE-168] Dedicated cloud Slack app: gitignore + internal setup doc

[singret]

Summary

• Adds /docs/slack-cloud-setup.md to .gitignore — internal ops guide, never committed to the public repo
• The setup doc covers: two-app structure (dev vs cloud), how to generate the production manifest from the UI, credential storage, token rotation, app branding checklist for App Directory submission, and per-developer dev app setup

Why a gitignored doc?

The manifest is already generated dynamically from window.location.origin — opening the Slack setup wizard on regen.fluidify.ai produces a manifest with production endpoints; on localhost it produces localhost URLs. No code change needed there.

The gitignored doc formalises the two-app separation so future developers don't accidentally share credentials between dev and prod.

Test plan

• git check-ignore docs/slack-cloud-setup.md returns the file (it does — confirmed locally)
• git status shows no untracked docs/slack-cloud-setup.md after creating it locally
• Review doc content matches acceptance criteria in OPE-168

[singret]

Closing — this only added a gitignore entry. The real OPE-168 work is multi-workspace OAuth for the cloud offering (Add to Slack flow, per-workspace token storage). Will be done in a proper implementation.