Slack dev guide: fix a heading that had drifted from its own section
Split Slack into a user page and a developer guide Slack.md was a hybrid. It is now the user-facing page — what it does, setting it up, the two traps that produce a working-looking result, the health check, and how it behaves. Slack-Developer-Guide.md takes the architecture: why it is not tracker #3, the bring-your-own-app reasoning, the kind-in-the-registry seam and its known debt, the file map, the four design decisions, and the six traps that only a live run exposed. Both validated: every wiki link resolves and every backticked path exists. Promoted eight bare filenames to full paths — the same thing that reads like a path but is not, caught by the same check as last time.