Generate Slack app manifest

Generate the Slack app manifest for a surface, with absolute API URLs (events webhook, interactivity webhook, and OAuth redirect), plus a link to Slack's app-creation page and a hosted connect-page handoff for the human credential + OAuth step. Paste the manifest into Slack's From a manifest option; Slack no longer accepts a manifest embedded in the URL. The redirect URL matches the OAuth start route byte-for-byte. When integrationId is omitted a pending Slack integration is reserved automatically (24h TTL) and returned as integrationId. A supplied integrationId must be a live pending reservation; an already-connected, revoked, or expired one is refused with 404, so omit it to get a fresh reservation. Credentials never pass through this route.