Issue #3597402: Remove the invalid auto-rebuild key from the Tugboat config

.tugboat/config.yml carried a root-level auto-rebuild: '0 4 * * *' key. Tugboat's config root only accepts services — there is no auto-rebuild key — so it was a no-op that implied a scheduled nightly rebuild that never happened.

Removes the dead key and replaces it with a comment noting that periodic base-preview rebuilds are a Tugboat dashboard/repository setting, not config (matching the convention used on sibling modules). No behaviour change; the config now validates to a single services root.