docs: apps.exec + compose troubleshooting appendix

Documents the new apps.exec tool alongside apps.logs and adds a
troubleshooting cookbook covering the common failure patterns we've
seen in the wild: missing migrations, silent apps.update reroutes,
502s on compose domains, healthcheck timeouts, and how to use
apps.exec as the platform's escape hatch for in-container inspection.

Bumps MCP version to 2.2.0 in the changelog and bumps the
vibn-frontend submodule to ship the apps.exec implementation.

Also includes setup-vibn-logs-user.sh (the script that installs the
locked-down SSH user on the Coolify host) which was already running
in production but not yet committed.

Made-with: Cursor

.gitignore

@@ -29,3 +29,4 @@
 **/.next/
 **/.turbo/
 **/coverage/
+.secrets/